@dc42 I enabled the pullup resistor for the io0.out pin, (and I found out a bad crimping contact for the X endstop... ^^').
Everthing works fine now. Thank you David.

@tkln I guess, that's not doable right now, but you could try sensorless homing for one of the motors on each tower.

Thanks everyone for your input!
I think I got it.
I removed the Z-Probe pin, and it now works completely.
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