It indeed seemed to be related to the ide cable. A second one solved the issue although it once occurred again as well. Therefor I assume it’s related to the cable, it’s shielding, possible interferences due to very non optimal cable-routing. Kabelführung.

Br

I assume this is a machine that uses an ISA based IO card, in which case cable can make a big difference when the IO card has no bus buffers. Without these buffers the system can get unreliable from excess cable capacitance that will be directly put on the ISA bus itself. This is one reason why I avoid bufferless cards in my own machines, but if you have no other choice, as short as possible IDE cable can help.

Yea that addresses the point

Swapping to the old power supply could be a quick fix worth trying. If that doesn't do the trick, we might need to dive deeper into the new case or connections

I indeed changed the PS upfront but without positive effect.