I had the same problem with my 3800 and using paper paper cut from a roll. I came to the conclusion that I had not cut the paper absolutely square, and that the printer was reading a smaller paper size than was there.
Not being square could explain my first failure because I just measured the paper leader with the Rotatrim and taped the leader to the back of the canvas. For the subsequent failures, I squared the leader, but the width (not length) of the leader was probably less than 1/2".
I would like to know how the printer senses the paper width, but my interest in the mechanism is now much reduced since I've been having started using a wider paper leader.