Hi,
at last i could find the reason for all my problems: the hot-end cooling fan didnt run fast enough. After years it was worn out, so it didnt cool the cooling rib enough and so the material was too hot above the hot end isolator and stuck there. When i rised the temperature it worked, because then the temperature in the isolator coupler was high enough to melt the material there.
Now i changed the fan (the last part after changing feeder, bowden, pt100, mainboard, heater, nozzle, isolator coupler, hot end isolator [some of these multiple times]) and it works!
Greets from the now very happy
loderunner