UM2 extrusion always stops

[manattic 0]

at first some prints starts well and then in the mid within one layer no extrusion anymore and the feeder is grinding in the material.

so i disassembled the feeder, cleaned it, tried a small print with no problems, but with a longer print extrusion stops.

so i disassembled the hot-end, and saw that the coupler was deformed (with only 250hrs on it, the last one was over 600hrs).

the nozzle was clean, so i put a new coupler in and started printing.

at hour 11 from a 12hrs print it happened again no extrusion anymore.

The feeder is just grinding and little to nothing comes out.

When doing an atomic method and push the material through i get a nice strand, but that does not help.