Octoprint serial errors related to running 2 USB cameras with Multicam or power problem

Camera model
Logitech C270 and generic endoscope

What is the problem?
I installed Multicam and got 2 cameras running on Pi 3B+

I started experiencing crashes: Octoprint would stop mid print and report that it could not establish exclusive serial connection.
Like this:
Octoprint stop. Hot end and bed would remain heated. Octoprint would report:

Offline (Error: SerialException: 'device reports readiness to read but returned no data (device disconnects or multiple access on port?)@comm.py:_readline:2823

Logs had lots of:

2020-04-23 16:46:12,596 - octoprint.util.comm - ERROR - Unexpected error while connecting to serial port: None SerialException: '[Errno 11] Could not exclusively lock port /dev/ttyUSB0: [Errno 11] Resource temporarily unavailable' @ comm.py:_openSerial:2691 (hook default)

(I hope I formatted this right)

Fussing with the setup showed that I got a lightning bolt while and after booting with both cameras installed.
I did some looking. Suspected power source or cables.
Replaced both.
Lightning bolt went away, but crashes persisted.

What did you already try to solve it?
Changed power cable
Changed power source to known good working 2.4A supply
eventually uninstalled Multicam plugin.
Seems to be printing now ok with one camera installed.

I am prepared to load cameras onto a separate Pi and serve them to the octoprint via network. In fact, I have a Zero W and a Pi 4 that I have not used for anything yet, and am itching for a project. Sounds fun.

I am, however, concerned that I might have damaged the SD card install and made it flaky with repeated crashing/power off or modifying the handling of webcams getting MultiCam going with 2 cameras. It is back working now, with MultiCam off, but that's not the goal of this whole thing. I'd like to get MultiCam running with 2 video streams.

I confess I previously had habit of just shutting the whole kaboodle down via remote plug when remote prints finished. I now know this is perilous. Then, I had multiple restarts due to a loose cable that I was bumping into while fiddling around with cabling, etc. That'll show up in the logs, I'm sure.

To make things even more fraught, I upgraded to Marlin 1.1.9 during all these events. It seems to be working perfectly, but that clouds things a bit in terms of troubleshooting. I don't think Marlin I likely the culprit here. The problem has gone away with deactivating Multicam.

I have printed multiple files successfully since.

Logs (/var/log/webcamd.log, syslog, dmesg, ... no logs, no support)

octoprint.log (78.9 KB)

I hope I did that right. Logs are larger than upload limit, so I selected a few instance where Pi starts up, runs octoprint, starts a print, experiences an error, starts again, etc.

If there is a better way to attach logs, please steer me. sorry if I goofed that up.
Additional information about your setup (OctoPrint version, OctoPi version, ...)

Pi 3B+
Octoprint 1.4.0
Printer is CR10S with Marlin 1.1.9

Thanks for any input. I really appreciate the help I've gotten in this support community. What a great resource. Thank you all for your effort.

Using one USB camera on a Pi is iffy enough but two? You're asking for trouble. Does it work without the USB cameras?

I spoke too soon, apparently. The problem persists.
Yesterday, I had multiple successful prints.
Overnight, the behavior repeated.
Octoprint stalled. Unexpected inability get a serial connection.
Octoprint_4-27_to_4-30_2020.log (2.2 MB)
Log is here. I see the failure happen, but I don't know what is causing it. Perhaps there is a clue in the log? I included a successful restart, through a print or two and then the failure.

Thanks for any help!
Dave