Looks like 1.41rc1 may have made things worse. I can't connect to my SKR 1.4 Turbo on /dev/ttyACM0, and it works fine through Pronterface on my laptop and sudo screen /dev/ttyACM0 on the Raspberry Pi running OctoPrint. I made multiple attempts to connect, using both auto-detect and explicit selection of /dev/ttyACM0 and a few different baud rates. The firmware was compiled with a baud rate of 250000, but I don't think that matters over an acm connection.
I've installed the "Remove ttyS Ports" plugin as well with no effect. Here's the serial.log output:
2020-07-07 08:23:00,668 - serial.log is currently not enabled, you can enable it via Settings > Serial Connection > Log communication to serial.log
2020-07-07 08:25:37,488 - serial.log is currently not enabled, you can enable it via Settings > Serial Connection > Log communication to serial.log
2020-07-07 08:31:58,817 - Enabling serial logging
2020-07-07 08:32:07,307 - Changing monitoring state from "Offline" to "Detecting serial connection"
2020-07-07 08:32:07,381 - Performing autodetection with 1 port/baudrate candidates: /dev/ttyACM0@250000
2020-07-07 08:32:07,384 - Connecting to port /dev/ttyACM0, baudrate 250000
2020-07-07 08:32:07,459 - Trying port /dev/ttyACM0, baudrate 250000
2020-07-07 08:32:07,461 - Changing monitoring state from "Detecting serial connection" to "Error: No more candidates to test, and no working port/baudrate combination detected."
2020-07-07 08:32:07,468 - Send: M112
2020-07-07 08:32:07,468 - Send: N1 M112*32
2020-07-07 08:32:07,469 - Send: N2 M104 T0 S0*35
2020-07-07 08:32:07,469 - Send: N3 M140 S0*102
2020-07-07 08:32:07,495 - Changing monitoring state from "Error: No more candidates to test, and no working port/baudrate combination detected." to "Offline (Error: No more candidates to test, and no working port/baudrate combination detected.)"
2020-07-07 08:32:07,508 - Connected to: None, starting monitor
2020-07-07 08:32:07,509 - Connection closed, closing down monitor
2020-07-07 08:32:20,022 - Changing monitoring state from "Offline" to "Opening serial connection"
2020-07-07 08:32:20,035 - Connecting to port /dev/ttyACM0, baudrate 250000
2020-07-07 08:32:20,064 - Changing monitoring state from "Opening serial connection" to "Connecting"
2020-07-07 08:32:20,067 - Connected to: Serial<id=0xa771e8f0, open=True>(port='/dev/ttyACM0', baudrate=250000, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitor
2020-07-07 08:32:20,075 - Send: N0 M110 N0*125
2020-07-07 08:32:50,073 - No answer from the printer within the connection timeout, trying another hello
2020-07-07 08:32:50,085 - Send: N0 M110 N0*125
2020-07-07 08:32:53,867 - Connection closed, closing down monitor
2020-07-07 08:32:53,871 - Changing monitoring state from "Connecting" to "Offline"
2020-07-07 08:33:05,111 - Changing monitoring state from "Offline" to "Opening serial connection"
2020-07-07 08:33:05,126 - Connecting to port /dev/ttyACM0, baudrate 115200
2020-07-07 08:33:05,144 - Changing monitoring state from "Opening serial connection" to "Connecting"
2020-07-07 08:33:05,152 - Connected to: Serial<id=0xa5371870, open=True>(port='/dev/ttyACM0', baudrate=115200, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitor
2020-07-07 08:33:05,158 - Send: N0 M110 N0*125
2020-07-07 08:33:35,164 - No answer from the printer within the connection timeout, trying another hello
2020-07-07 08:33:35,176 - Send: N0 M110 N0*125
2020-07-07 08:33:45,167 - There was a timeout while trying to connect to the printer
2020-07-07 08:33:45,179 - Changing monitoring state from "Connecting" to "Offline"
2020-07-07 08:33:45,185 - Connection closed, closing down monitor
EDIT: And now I'm completely down the rabbit hole. I thought it might be a Raspberry Pi 4 incompatibility, but swapping the SD card back to my Pi 3B+ resulted in an unbootable image that no longer worked in either unit. I've imaged a fresh SD card with OctoPi and I'm now back at square one with 1.3.12, which does connect just fine.
