So finally I managed to assemble the printer. But all my testprints of Benchy and a Lowrider Benchy look a bit strange.
In Cura all models look normal.
The magnetic plate is fixed with clamps and does not shift during the print.
Where is the mistake?
Trimpot adjustment should only be made with printer powered off. If adjusted whilst powered on, especially with a metal screw driver, the board will get fried.