What is the problem?
I am having problems getting the USB serial connection working between the Raspberry PI and the Ender 3 Pro. When I click on the "Connect" button, the Ender UI screen seems to reset and I get a status message that states: " Error: Connection error, see Terminal tab" The errors in the Terminal Tab seem to be a timeout issue:
Changing monitoring state from "Offline" to "Detecting serial port"
Serial port list: ['/dev/ttyUSB0', '/dev/ttyUSB0', '/dev/ttyUSB0']
Trying /dev/ttyUSB0
Changing monitoring state from "Detecting serial port" to "Error: Connection error, see Terminal tab"
Unexpected error while connecting to serial port: AUTO IspError: ''Timeout'' @ comm.py:_openSerial:2611 (hook default)
What did you already try to solve it?
I have tried two different Raspberry PI (Model B+ and B+ V2).
I have tried different USB cables
I tried printing that little thing to isolate +5V
I have tried connecting with the Ender 3 power on and off
I opened up one of my USB cables and cut the 5 V to isolate it
I tried pushing the button 500 times in a row 
I tried a number of different power-on sequences and plug-in sequences.
None of the above have seemed to change what is happening
Logs (octoprint.log, serial.log or output on terminal tab, ...)
octoprint.log (181.4 KB)
serial.log (938 Bytes)
Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, ...)
I am running OctoPrint 1.3.11
OctoPi Version 0.16.0, running on Raspberry Pi 3 Model B Rev 1.2
I just bought the Ender 3 Pro, not sure what FW it actually has, but I haven't changed the firmware yet.