Thanks a lot, Raby. Since I built the kit, I forgot the driver voltage adjustment.
I checked now the driver voltages and I think that over the time (I have the printer now 2 years), maybe through vibrations, the potentiometers changed their values. Otherwise I can not explain, why the voltage changed.
The extruder motor driver even was at 1,3 V! Wonder that it still worked. I adjusted it again in the demanded ranges.
Now the printer runs again very stable.
That proves again how useful this forum is.