Cricket, look for something ON the bobbincase that's grabbing thread.
Here's my theory: Twice the bobbin case has been yanked out of its position. I don't think this is really supposed to be possible in regular use. Not with that retention system. The only way I could see it dislodging would be with a giant yank to the back and upward. Pretty much what the take-up lever would be doing to the thread that's probably yanking the case out of place. The other place I would check is on the thread path to the right side of the take up lever. If it were caught there, and the TUL wanted thread delivered from the right side, it may yank the case out of place while trying to grab thread from the other side. Does it pucker the seam at the same time?
You could also try the throat plate and bobbin case from your 401. Heck, even the tensioner. That could eliminate 3 places for burrs.