Well, actually, I don't think it is stop. When printing is about to start, the extruder goes down, then, 5-10 cm from the stop, it stops and begins "printing" (just like that - in the air).

Same soft is used on an identical printer, so it is not softwatre.

Send M119 to see what the endstops are doing.

Seems it was RAMPS/Arduino seeing noise instead of sensors. Fortunately, I had another Arduino.