Paper accordion fold jamming in HP LJ Pro 400 (M451)

I have been having ongoing issues with attempting to fix this printer for a couple weeks now. I don't get any error codes beyond there being a jam in the rear door, where the paper is accordion folding between the ITB and the transfer roller. I have already tried to replace the ITB itself, as well as checking to see if the motor is running properly (it seems to be) with no dice.

It was suggested to me on another forum that the issue might be the registration assembly itself (either a sensor or other issue) but thus far, I have not been able to even remove it to see or replace, as there are no easily available instructions on how to do so. I tried going in on my own, and ran into the problem where I cannot figure out how to remove other assemblies that are in the way (one of which looks like it might need a phillips head bent like an alan wrench to undo.)

Any assistance or recommendations would be very helpful.

-Greg