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.