I forgot to inform you that I tested another 5.2 board I have, this one did not power on as strangely as this one does so there is definitely a malfunction, definitely not the power supply.
However, the other board also displayed more or less similar behavious, just much less extreme. So it must be in the design (an error ?) of this particular model, but the one that has the extreme behaviour when powering on must have some kind of malfunction. I was planning to replace the SG3524 and TL074 in the power supply section just as a wild guess, but since the other board also displayed a power-on different than most Rubicons I have decided to let it be and put it in my car. If at some point the malfunction becomes it might be easier to find the cause.