From your octoprint.log.
This issue happens just after Changing monitoring state from "Connecting" to "Operational"
I have the same printer here too. I've modified a USB cable and disconnected the 5V wire. It runs more secure because the printer draws current from the 5V rail of the USB.
That can make issues with the USB of the Pi.
In detail: instead
M110 N0*125
the printer received this:
"0ok N0 P15 B3
And that is no good.
Thank you so much for checking and comparing logs!
Well, it's settled, the 5V pin has to go 
Did you have problems as well with your T-Rex then I assume?
Not in that way, but when the Pi with OctoPrint was connected to off powered printer, the display was still lit and I got the under voltage issue message in OctoPrint.
Else the the printer runs fine. I only hat to adjust the nozzle height of the right extruder, it was a bit too high.
I'm generally happy but I cant fine tune that last little thing to satisfy me. It seems to me that it cant handle retractions very well, or it's just my slicer settings...
Have you tried the latest Marlin FW?
I want to try but I'm unsure of how to do it. I have downloaded arduino and try to read as much as I can on how the configuration shall be.
Here is the marlin example config for the Trex 3
Nice! 
I'm thinking about to install it.
The 5V pin is in the trash, literally.
Thanks again and hopefully this will be the end if this.
/Micke
Just thought I would share my situation. I updated from the Creality firmware to the Marlin 2.0.X bug fixes branch, and immediately started getting the communication connection issue. Tried all of the published suggestions...remove extensions, handshake setting in octoprint, new Pi, reflashed to new SD...nothing worked. Rolled back to the 2.0.7 release of Marlin, and the communication issue was gone.
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