Okay sorry guys I just found the problem even though I dont know how this could be.
It seems like the grounding point was not strong enough. I measured the VIN and it was only around 2-3V. So I used another ground point and not it works.
The old ground point was directly connected to the ground of the plug that goes into the printer. Now I connected it to the ground coming out of the power supply.
I did not knew that there could be any difference?
I mean the ground point from the plug also goes into the power supply.
But now it works, thank you anyways @jay_s_uk !