After updating to the latest octoprint version that was released a couple of days ago i'm getting the following error msg when trying to connect to the printer:
Your printer's firmware reported an error. Due to that OctoPrint will disconnect. Reported error: Unsupported command - oooook
I'm running an MKS SBASE 1.3 with Smoothieware.
I recognize the "oooook" from the terminal, but it has worked just fine in earlier versions.
Logs pls 
especially the serial log
2019-10-23 11:55:50,269 - octoprint.startup - INFO - ******************************************************************************
2019-10-23 11:55:50,271 - octoprint.startup - INFO - Starting OctoPrint 1.3.12
2019-10-23 11:55:50,276 - octoprint.startup - INFO - ******************************************************************************
2019-10-23 11:55:50,351 - octoprint.startup - INFO - We don't appear to be online, not fetching plugin blacklist
2019-10-23 11:55:50,430 - octoprint.plugin.core - INFO - Loading plugins from /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins, /home/pi/.octoprint/plugins and installed plugin packages...
2019-10-23 11:55:54,491 - octoprint.plugin.core - INFO - Plugin Bed Visualizer (0.1.9) is disabled.
2019-10-23 11:55:54,541 - octoprint.plugin.core - INFO - Plugin Action Commands (0.1) is disabled.
2019-10-23 11:55:54,560 - octoprint.plugin.core - INFO - Found 20 plugin(s) providing 18 mixin implementations, 26 hook handlers
2019-10-23 11:55:54,954 - octoprint.server.heartbeat - INFO - Starting server heartbeat, 900.0s interval
2019-10-23 11:55:55,012 - octoprint.server - INFO - Intermediary server started
2019-10-23 11:55:55,013 - octoprint.plugin.core - INFO - Loading plugins from /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins, /home/pi/.octoprint/plugins and installed plugin packages...
2019-10-23 11:55:56,012 - octoprint.plugin.core - INFO - Found 20 plugin(s) providing 18 mixin implementations, 26 hook handlers
2019-10-23 11:55:56,119 - octoprint.filemanager.storage - INFO - Initializing the file metadata for /home/pi/.octoprint/uploads...
2019-10-23 11:55:57,427 - octoprint.filemanager.storage - INFO - ... file metadata for /home/pi/.octoprint/uploads initialized successfully.
2019-10-23 11:55:57,698 - octoprint.plugins.softwareupdate - INFO - Loaded version cache from disk
2019-10-23 11:56:00,107 - octoprint.util.pip - INFO - Using "/home/pi/oprint/bin/python2 -m pip" as command to invoke pip
2019-10-23 11:56:01,633 - octoprint.util.pip - INFO - Version of pip is 19.0.1
2019-10-23 11:56:01,635 - octoprint.util.pip - INFO - pip installs to /home/pi/oprint/lib/python2.7/site-packages (writable -> yes), --user flag needed -> no, virtual env -> yes
2019-10-23 11:56:01,636 - octoprint.util.pip - INFO - ==> pip ok -> yes
2019-10-23 11:56:01,641 - octoprint.plugin.core - INFO - Initialized 18 plugin implementation(s)
2019-10-23 11:56:01,655 - octoprint.plugin.core - INFO - 20 plugin(s) registered with the system:
| Action Command Prompt Support (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/action_command_prompt
| !Action Commands (0.1) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_actioncommands
| Announcement Plugin (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/announcements
| Anonymous Usage Tracking (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/tracking
| Application Keys Plugin (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/appkeys
| Backup & Restore (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/backup
| !Bed Visualizer (0.1.9) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_bedlevelvisualizer
| BLTouch Plugin (0.3.3) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_BLTouch
| Core Wizard (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/corewizard
| Custom Control Editor (0.2.1) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_customControl
| Discovery (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/discovery
| Error Tracking (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/errortracking
| Force Login (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/forcelogin
| Logging (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/logging
| Pi Support Plugin (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/pi_support
| Plugin Manager (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/pluginmanager
| Printer Safety Check (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/printer_safety_check
| Software Update (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/softwareupdate
| Virtual Printer (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/virtual_printer
| Webcam Tab (0.1.2) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_webcamtab
2019-10-23 11:56:01,677 - octoprint.environment - INFO - Detected environment is Python 2.7.13 under Linux (linux2). Details:
| hardware:
| cores: 4
| freq: 1400.0
| ram: 918188032
| os:
| id: linux
| platform: linux2
| plugins:
| pi_support:
| model: Raspberry Pi 3 Model B Plus Rev 1.3
| octopi_version: 0.16.0
| python:
| pip: 19.0.1
| version: 2.7.13
| virtualenv: /home/pi/oprint
2019-10-23 11:56:01,691 - octoprint.server - INFO - Reset webasset folder /home/pi/.octoprint/generated/webassets...
2019-10-23 11:56:01,708 - octoprint.server - INFO - Reset webasset folder /home/pi/.octoprint/generated/.webassets-cache...
2019-10-23 11:56:02,214 - octoprint.server - INFO - Shutting down intermediary server...
2019-10-23 11:56:02,392 - octoprint.server - INFO - Intermediary server shut down
2019-10-23 11:56:02,395 - octoprint.events - INFO - Processing startup event, this is our first event
2019-10-23 11:56:02,395 - octoprint.events - INFO - Adding 0 events to queue that were held back before startup event
2019-10-23 11:56:02,398 - octoprint.filemanager - INFO - Adding backlog items from all storage types to analysis queue...
2019-10-23 11:56:02,402 - octoprint.filemanager - INFO - Added 0 items from storage type "local" to analysis queue
2019-10-23 11:56:02,413 - octoprint.server.util.watchdog - INFO - Running initial scan on watched folder...
2019-10-23 11:56:02,415 - octoprint.server.util.watchdog - INFO - ... initial scan done.
2019-10-23 11:56:02,427 - octoprint.plugins.discovery - INFO - Registered 'OctoPrint instance on octopi' for _http._tcp
2019-10-23 11:56:02,440 - octoprint.plugins.discovery - INFO - Registered 'OctoPrint instance on octopi' for _octoprint._tcp
2019-10-23 11:56:02,452 - octoprint.plugins.discovery - INFO - Registered OctoPrint instance on octopi for SSDP
2019-10-23 11:56:02,463 - octoprint.plugins.pi_support - WARNING - This Raspberry Pi is reporting problems that might lead to bad performance or errors caused by overheating or insufficient power.
!!! UNDERVOLTAGE REPORTED !!! Make sure that the power supply and power cable are capable of supplying enough voltage and current to your Pi.
2019-10-23 11:56:02,468 - octoprint.server - INFO - Listening on http://127.0.0.1:5000
2019-10-23 11:56:02,507 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 24.00ms
2019-10-23 11:56:02,538 - octoprint.plugins.pluginmanager - INFO - Loaded plugin repository data from disk, was still valid
2019-10-23 11:56:04,044 - octoprint.plugins.pluginmanager - INFO - Loaded notice data from disk, was still valid
2019-10-23 11:56:05,903 - octoprint.util.pip - INFO - Using "/home/pi/oprint/bin/python2 -m pip" as command to invoke pip
2019-10-23 11:56:05,905 - octoprint.util.pip - INFO - pip installs to /home/pi/oprint/lib/python2.7/site-packages (writable -> yes), --user flag needed -> no, virtual env -> yes
2019-10-23 11:56:05,905 - octoprint.util.pip - INFO - ==> pip ok -> yes
2019-10-23 11:56:05,932 - octoprint.util.connectivity_checker - INFO - Connectivity changed from offline to online
2019-10-23 11:56:05,959 - octoprint.plugins.pluginmanager - INFO - Loaded plugin repository data from disk, was still valid
2019-10-23 11:56:06,493 - octoprint.plugins.pluginmanager - INFO - Loaded notice data from disk, was still valid
2019-10-23 11:56:06,958 - octoprint.server.preemptive_cache - INFO - Preemptively caching / (ui _default) for {'query_string': 'l10n=en', 'path': '/', 'base_url': 'http://192.168.1.12/'}
2019-10-23 12:22:57,422 - octoprint.server.heartbeat - INFO - Server heartbeat <3
2019-10-23 12:23:09,634 - octoprint.server.preemptive_cache - INFO - ... done in 17.02s
2019-10-23 12:23:12,072 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 19.93ms
2019-10-23 12:23:12,136 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 18.80ms
2019-10-23 12:23:12,239 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.43ms
2019-10-23 12:23:12,294 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.67ms
2019-10-23 12:23:12,349 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 10.00ms
2019-10-23 12:23:12,980 - octoprint.server.util.flask - INFO - Passively logging in user jontek2 from ::ffff:192.168.1.9
2019-10-23 12:23:14,411 - octoprint.server.util.sockjs - INFO - New connection from client: ::ffff:192.168.1.9
2019-10-23 12:23:14,701 - octoprint.server.util.flask - INFO - Passively logging in user jontek2 from ::ffff:192.168.1.9
2019-10-23 12:23:16,331 - octoprint.server.util.sockjs - INFO - User jontek2 logged in on the socket from client ::ffff:192.168.1.9
2019-10-23 12:23:19,101 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 17.39ms
2019-10-23 12:23:19,220 - octoprint.util.comm - INFO - Changing monitoring state from "Offline" to "Opening serial port"
2019-10-23 12:23:19,225 - octoprint.util.comm - INFO - Changing monitoring state from "Opening serial port" to "Connecting"
2019-10-23 12:23:19,240 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 9.30ms
2019-10-23 12:23:19,245 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 12:23:19,248 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 12:23:19,256 - octoprint.util.comm - INFO - Changing monitoring state from "Connecting" to "Operational"
2019-10-23 12:23:19,265 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 12:23:19,277 - octoprint.util.comm - INFO - Printer reports firmware name "Smoothieware,"
2019-10-23 12:24:43,125 - octoprint.startup - INFO - ******************************************************************************
2019-10-23 12:24:43,127 - octoprint.startup - INFO - Starting OctoPrint 1.3.12
2019-10-23 12:24:43,132 - octoprint.startup - INFO - ******************************************************************************
2019-10-23 12:24:43,213 - octoprint.startup - INFO - We don't appear to be online, not fetching plugin blacklist
2019-10-23 12:24:43,299 - octoprint.plugin.core - INFO - Loading plugins from /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins, /home/pi/.octoprint/plugins and installed plugin packages...
2019-10-23 12:24:45,845 - octoprint.plugin.core - INFO - Plugin Bed Visualizer (0.1.9) is disabled.
2019-10-23 12:24:45,893 - octoprint.plugin.core - INFO - Plugin Action Commands (0.1) is disabled.
2019-10-23 12:24:45,912 - octoprint.plugin.core - INFO - Found 20 plugin(s) providing 18 mixin implementations, 26 hook handlers
2019-10-23 12:24:46,868 - octoprint.server.heartbeat - INFO - Starting server heartbeat, 900.0s interval
2019-10-23 12:24:46,970 - octoprint.server - INFO - Intermediary server started
2019-10-23 12:24:46,971 - octoprint.plugin.core - INFO - Loading plugins from /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins, /home/pi/.octoprint/plugins and installed plugin packages...
2019-10-23 12:24:48,730 - octoprint.plugin.core - INFO - Found 20 plugin(s) providing 18 mixin implementations, 26 hook handlers
2019-10-23 12:24:48,831 - octoprint.filemanager.storage - INFO - Initializing the file metadata for /home/pi/.octoprint/uploads...
2019-10-23 12:24:49,608 - octoprint.filemanager.storage - INFO - ... file metadata for /home/pi/.octoprint/uploads initialized successfully.
2019-10-23 12:24:49,727 - octoprint.plugins.softwareupdate - INFO - Loaded version cache from disk
2019-10-23 12:24:51,656 - octoprint.util.pip - INFO - Using "/home/pi/oprint/bin/python2 -m pip" as command to invoke pip
2019-10-23 12:24:53,126 - octoprint.util.pip - INFO - Version of pip is 19.0.1
2019-10-23 12:24:53,127 - octoprint.util.pip - INFO - pip installs to /home/pi/oprint/lib/python2.7/site-packages (writable -> yes), --user flag needed -> no, virtual env -> yes
2019-10-23 12:24:53,128 - octoprint.util.pip - INFO - ==> pip ok -> yes
2019-10-23 12:24:53,134 - octoprint.plugin.core - INFO - Initialized 18 plugin implementation(s)
2019-10-23 12:24:53,147 - octoprint.plugin.core - INFO - 20 plugin(s) registered with the system:
| Action Command Prompt Support (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/action_command_prompt
| !Action Commands (0.1) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_actioncommands
| Announcement Plugin (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/announcements
| Anonymous Usage Tracking (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/tracking
| Application Keys Plugin (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/appkeys
| Backup & Restore (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/backup
| !Bed Visualizer (0.1.9) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_bedlevelvisualizer
| BLTouch Plugin (0.3.3) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_BLTouch
| Core Wizard (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/corewizard
| Custom Control Editor (0.2.1) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_customControl
| Discovery (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/discovery
| Error Tracking (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/errortracking
| Force Login (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/forcelogin
| Logging (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/logging
| Pi Support Plugin (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/pi_support
| Plugin Manager (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/pluginmanager
| Printer Safety Check (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/printer_safety_check
| Software Update (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/softwareupdate
| Virtual Printer (bundled) = /home/pi/oprint/lib/python2.7/site-packages/octoprint/plugins/virtual_printer
| Webcam Tab (0.1.2) = /home/pi/oprint/local/lib/python2.7/site-packages/octoprint_webcamtab
2019-10-23 12:24:53,170 - octoprint.environment - INFO - Detected environment is Python 2.7.13 under Linux (linux2). Details:
| hardware:
| cores: 4
| freq: 1400.0
| ram: 918188032
| os:
| id: linux
| platform: linux2
| plugins:
| pi_support:
| model: Raspberry Pi 3 Model B Plus Rev 1.3
| octopi_version: 0.16.0
| python:
| pip: 19.0.1
| version: 2.7.13
| virtualenv: /home/pi/oprint
2019-10-23 12:24:53,184 - octoprint.server - INFO - Reset webasset folder /home/pi/.octoprint/generated/webassets...
2019-10-23 12:24:53,202 - octoprint.server - INFO - Reset webasset folder /home/pi/.octoprint/generated/.webassets-cache...
2019-10-23 12:24:53,715 - octoprint.server - INFO - Shutting down intermediary server...
2019-10-23 12:24:54,125 - octoprint.server - INFO - Intermediary server shut down
2019-10-23 12:24:54,128 - octoprint.events - INFO - Processing startup event, this is our first event
2019-10-23 12:24:54,129 - octoprint.events - INFO - Adding 0 events to queue that were held back before startup event
2019-10-23 12:24:54,131 - octoprint.filemanager - INFO - Adding backlog items from all storage types to analysis queue...
2019-10-23 12:24:54,134 - octoprint.filemanager - INFO - Added 0 items from storage type "local" to analysis queue
2019-10-23 12:24:54,135 - octoprint.server.util.watchdog - INFO - Running initial scan on watched folder...
2019-10-23 12:24:54,147 - octoprint.server.util.watchdog - INFO - ... initial scan done.
2019-10-23 12:24:54,161 - octoprint.plugins.discovery - INFO - Registered 'OctoPrint instance on octopi' for _http._tcp
2019-10-23 12:24:54,173 - octoprint.plugins.discovery - INFO - Registered 'OctoPrint instance on octopi' for _octoprint._tcp
2019-10-23 12:24:54,179 - octoprint.plugins.discovery - INFO - Registered OctoPrint instance on octopi for SSDP
2019-10-23 12:24:54,198 - octoprint.plugins.pi_support - WARNING - This Raspberry Pi is reporting problems that might lead to bad performance or errors caused by overheating or insufficient power.
!!! UNDERVOLTAGE REPORTED !!! Make sure that the power supply and power cable are capable of supplying enough voltage and current to your Pi.
2019-10-23 12:24:54,204 - octoprint.server - INFO - Listening on http://127.0.0.1:5000
2019-10-23 12:24:54,243 - octoprint.plugins.pluginmanager - INFO - Loaded plugin repository data from disk, was still valid
2019-10-23 12:24:54,922 - octoprint.plugins.pluginmanager - INFO - Loaded notice data from disk, was still valid
2019-10-23 12:24:56,792 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 46.20ms
2019-10-23 12:24:56,855 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 20.02ms
2019-10-23 12:24:57,338 - octoprint.util.pip - INFO - Using "/home/pi/oprint/bin/python2 -m pip" as command to invoke pip
2019-10-23 12:24:57,341 - octoprint.util.pip - INFO - pip installs to /home/pi/oprint/lib/python2.7/site-packages (writable -> yes), --user flag needed -> no, virtual env -> yes
2019-10-23 12:24:57,342 - octoprint.util.pip - INFO - ==> pip ok -> yes
2019-10-23 12:24:57,383 - octoprint.util.connectivity_checker - INFO - Connectivity changed from offline to online
2019-10-23 12:24:57,442 - octoprint.plugins.pluginmanager - INFO - Loaded plugin repository data from disk, was still valid
2019-10-23 12:24:58,061 - octoprint.plugins.pluginmanager - INFO - Loaded notice data from disk, was still valid
2019-10-23 12:24:58,552 - octoprint.server.preemptive_cache - INFO - Preemptively caching / (ui _default) for {'query_string': 'l10n=en', 'path': '/', 'base_url': 'http://192.168.1.12/'}
2019-10-23 13:28:51,458 - octoprint.server.heartbeat - INFO - Server heartbeat <3
2019-10-23 13:29:02,283 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 42.08ms
2019-10-23 13:29:02,429 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 9.87ms
2019-10-23 13:29:02,511 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 10.40ms
2019-10-23 13:29:02,566 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 9.17ms
2019-10-23 13:29:02,621 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 9.47ms
2019-10-23 13:29:03,554 - octoprint.server.preemptive_cache - INFO - ... done in 17.69s
2019-10-23 13:29:15,069 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 18.91ms
2019-10-23 13:29:15,133 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 18.92ms
2019-10-23 13:29:30,007 - octoprint.server.util.flask - INFO - Passively logging in user jontek2 from ::ffff:192.168.1.9
2019-10-23 13:29:31,756 - octoprint.server.util.sockjs - INFO - New connection from client: ::ffff:192.168.1.9
2019-10-23 13:29:32,131 - octoprint.server.util.flask - INFO - Passively logging in user jontek2 from ::ffff:192.168.1.9
2019-10-23 13:29:33,192 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.49ms
2019-10-23 13:29:33,297 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.02ms
2019-10-23 13:29:33,352 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 9.13ms
2019-10-23 13:29:33,402 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.03ms
2019-10-23 13:29:33,455 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.05ms
2019-10-23 13:29:34,130 - octoprint.server.util.sockjs - INFO - User jontek2 logged in on the socket from client ::ffff:192.168.1.9
2019-10-23 13:29:46,063 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 17.27ms
2019-10-23 13:29:46,132 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 17.47ms
2019-10-23 13:30:04,064 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 18.10ms
2019-10-23 13:30:04,131 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 20.07ms
2019-10-23 13:30:04,234 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.40ms
2019-10-23 13:30:04,289 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.36ms
2019-10-23 13:30:04,340 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 8.15ms
2019-10-23 13:30:17,066 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 18.82ms
2019-10-23 13:30:17,134 - tornado.access - WARNING - 409 GET /api/printer (::ffff:192.168.1.99) 26.44ms
2019-10-23 13:30:18,467 - octoprint.util.comm - INFO - Changing monitoring state from "Offline" to "Opening serial port"
2019-10-23 13:30:18,471 - octoprint.util.comm - INFO - Changing monitoring state from "Opening serial port" to "Connecting"
2019-10-23 13:30:18,477 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 13:30:18,479 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 13:30:18,483 - octoprint.util.comm - WARNING - Received an error from the printer's firmware: Unsupported command - oooook
| Last lines in terminal:
| Connecting to: /dev/ttyACM0
| Changing monitoring state from "Offline" to "Opening serial port"
| Connected to: Serial<id=0x6a5974f0, open=True>(port='/dev/ttyACM0', baudrate=115200, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitor
| Changing monitoring state from "Opening serial port" to "Connecting"
| Send: N0 M110 N0*125
| Recv: error:Unsupported command - oooook
2019-10-23 13:30:18,484 - octoprint.util.comm - INFO - Changing monitoring state from "Connecting" to "Error: Unsupported command - oooook"
2019-10-23 13:30:18,488 - octoprint.util.comm - INFO - Force-sending M112 to the printer
2019-10-23 13:30:18,499 - octoprint.util.comm - INFO - Changing monitoring state from "Error: Unsupported command - oooook" to "Offline (Error: Unsupported command - oooook)"
2019-10-23 13:30:29,443 - octoprint.util.comm - INFO - Changing monitoring state from "Offline" to "Opening serial port"
2019-10-23 13:30:29,447 - octoprint.util.comm - INFO - Changing monitoring state from "Opening serial port" to "Connecting"
2019-10-23 13:30:29,455 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 13:30:29,458 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 13:30:29,463 - octoprint.util.comm - INFO - Changing monitoring state from "Connecting" to "Operational"
2019-10-23 13:30:29,472 - octoprint.util.comm - INFO - M110 detected, setting current line number to 0
2019-10-23 13:30:29,481 - octoprint.util.comm - INFO - Printer reports firmware name "Smoothieware,"
2019-10-23 13:30:29,487 - octoprint.util.comm - WARNING - Received an error from the printer's firmware:
| Last lines in terminal:
| Connecting to: /dev/ttyACM0
| Changing monitoring state from "Offline" to "Opening serial port"
| Connected to: Serial<id=0x6a50a4b0, open=True>(port='/dev/ttyACM0', baudrate=115200, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitor
| Changing monitoring state from "Opening serial port" to "Connecting"
| Send: N0 M110 N0*125
| Send: N0 M110 N0*125
| Recv: ok
| Changing monitoring state from "Connecting" to "Operational"
| Recv: ok
| Send: N0 M110 N0*125
| Send: N1 M115*39
| Recv: ok
| Send: N2 M21*18
| Recv: FIRMWARE_NAME:Smoothieware, FIRMWARE_URL:http%3A//smoothieware.org, X-SOURCE_CODE_URL:https://github.com/Smoothieware/Smoothieware, FIRMWARE_VERSION:edge-5829d90, X-FIRMWARE_BUILD_DATE:Mar 3 2019 14:54:42, X-SYSTEM_CLOCK:100MHz, X-AXES:5, X-GRBL_MODE:0, X-CNC:0, X-MSD:1, X-WARNING:This is not a sanctioned board and may be unreliable and even dangerous. This MCU is deprecated, and cannot guarantee proper function
| Recv:
| Recv: ok
| Recv: !!
2019-10-23 13:30:29,489 - octoprint.util.comm - INFO - Changing monitoring state from "Operational" to "Error: "
2019-10-23 13:30:29,491 - octoprint.util.comm - INFO - Force-sending M112 to the printer
2019-10-23 13:30:29,500 - octoprint.util.comm - INFO - Changing monitoring state from "Error: " to "Offline (Error: )"
I guess it's a malformed ok which might be caused by this
Do you have a sufficient psu?
Also your usb cable could be a problem. Test with another one with a ferrite bead if possible.
And please send the serial.log next time
But I think we got the necessary information.
Undervoltage can cause all kinds of problems and you should fix that first.
Unfortunatly it's not caused by undervoltage. It was just coincidence. Got same error without undervoltage.
Also just got this error:
Your printer's firmware reported an error. Due to that OctoPrint will disconnect. Reported error: Unsupported command - ooN0
Didnt have serial.log enabled, will report next time i get an error.
OK got the error again, this time with serial.log:
2019-10-23 20:16:19,729 - Connecting to: /dev/ttyACM0
2019-10-23 20:16:19,747 - Changing monitoring state from "Offline" to "Opening serial port"
2019-10-23 20:16:19,752 - Connected to: Serial<id=0x73249cd0, open=True>(port='/dev/ttyACM0', baudrate=115200, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitor
2019-10-23 20:16:19,752 - Changing monitoring state from "Opening serial port" to "Connecting"
2019-10-23 20:16:19,763 - Send: N0 M110 N0*125
2019-10-23 20:16:19,766 - Send: N0 M110 N0*125
2019-10-23 20:16:19,768 - Recv: error:Unsupported command - oooook
2019-10-23 20:16:19,772 - Changing monitoring state from "Connecting" to "Error: Unsupported command - oooook"
2019-10-23 20:16:19,781 - Send: M112
2019-10-23 20:16:19,781 - Send: N1 M112*32
2019-10-23 20:16:19,783 - Send: N2 M104 T0 S0*35
2019-10-23 20:16:19,784 - Send: N3 M140 S0*102
2019-10-23 20:16:19,798 - Changing monitoring state from "Error: Unsupported command - oooook" to "Offline (Error: Unsupported command - oooook)"
2019-10-23 20:16:19,805 - Connection closed, closing down monitor
And this time there's no undervoltage 
Did you see this? Seconding this.
The undervoltage warning is an issue that reports a condition of the RasPi and will not occur in the serial.log, that exclusively only logs the communication between the RasPi and the printer.
No, i missed that. Currently don't have another cable but will get a new one tomorrow and report back.
Got it! But i don't think this error is related to undervoltage.
I strongly have the suspicion that there is maybe something wrong with the firmware on the MKS board.
It might also behave like this if for some unknown reason the firmware isn't using 8N1 for the specifics of the serial connection. Perhaps also, the serial cable's male ends aren't tightly fitting in their respective connectors.
The cable is fitted nicely and there's a tight connection.
However, i am using on of theese blue cables that you get from China which i suspect is not the highest quality. I'll buy a new high quality cable today and report back.
New cable is installed and no errors so far! Will do some testing but for now it's looking promising!
Internet rule #412: All cables come from China.
But the blue cables are more chinese than the black ones 
Haha
With the new, black cable (!!), it's working just fine. Thanks everyone!
:throws out all blue cables: 