I’d be really interested to hear more of this story! How you isolated the problem down to the level of the PSU is going deeper into the machine than I’ve personally been, so this story could be a great teacher.
It sounds way cooler than it was. For some reason I openened up the server box and during a reboot I saw the LED on the motherboard flickering in a way I didn't expect. So we tried to change PSU and then everything worked.