When printing TPU, my printer recently had repeatedly dropouts, after pulling out the filament was wrapped: I would not have thought of the cause at first ...

The problem area

Based on the distance or how far the filament protrudes into the MK8 extruder until it forms the node, the problem area seemed to be somewhere in the middle. i.e., not quite at the hotend, but in front of it. So I unscrewed the nozzle and used a screw to get the PTFE-Teflon tube out.

Bingo! The Teflon tube looks more than used up:

The inner wall thickness is no longer original, the part has dissolved in the middle: crass

In the bulge, there is exactly enough space for the wrapped filament:

It is interesting, however, that I had no issue with PETG and PLA when printing, only TPU has last completely wedged.

How come?

The Teflon tubing should get more friction in the middle. I can only explain it this way: The filament is already relatively liquid in the direction of the nozzle, but at the beginning of the tube is still rather solid. So it still has its original shape, then in the middle it is pressed apart and is firm enough to slowly rub away the wall?