Hey, so, not really looking for troubleshooting as we've fixed the problem but I thought maybe a brief summary of our symptoms/issue could help someone else should they come across this and we all know how helpful it can be to find the same problem you're having in a place like this, so here it is:
Basically, we were getting arithmetic errors on our external encoder, sporadically, for a while. Whatever, reset and continue on with the welding; until, a few weeks back we get a 4107 Major - that's basically an encoder's position not lining up with where the controller says it should be.
The first reboot of the controller cleared it, but after that we effectively are locked out of the machine. Upon inspection of the inside of the controller, the PWM power feed cable was... for lack of a better word, *scorched*. The weaved casing is melted and just, gnarly-looking in general. But everything else was... perfectly fine? Very strange.
I'll save you the myriad things that we did to diagnose the issue but as it turned out - the cable that ran inside of the manipulator arm, well, it's harness had a rub-point that over time, I guess, wore itself right raw to the live wire inside, which subsequently made contact with the arm itself, which I guess meant that an absurd surge of power connected with the arm, which ran down the ground, which was connected to the external rotary positioner, which decided that its best route of travel was through its encoder and into the aforementioned scorched cable.
Wow. Like, so what caught our attention was a nickel-sized perfect circle on the arm that was missing paint that was so perfectly formed that it was a spot that I guess on initial inspection we just, always thought was part of the paint job. That's where the short happened, and the wire welded itself to the manipulator on the inside at that position.
Right, so, a service call, a new encoder, a bunch of housing/harness and a PWM cable later, the problem is remedied and we wait for the next one. But if you find yourself with confusingly-nuked cable housing and encoder errors on an external axis, there is a reasonable yet tiny chance that your robot has welded itself... to itself.