I redid the wiring completely on all those stepper motors. So I have to assume, this was the issue. v(ツ)v
This is total speculation, but I think the resistance of the cables over the distance (1.2m) was to high, proportional to cable diameter (Ø=0.8mm) and the containing conductor (aluminium). So I used some HDMI cables, because of the electromagnetic isolation of the wires and the copper. I didn’t have any undervoltage or layershifts since.
You don’t have to use that, but - as a tipp - try exchanging the wires first.