Thanks! The whole PC had a time (when its age was ~20 years) where it still booted, but with reset BIOS settings, followed by a time where it doesn’t boot up anymore. So I believe the most likely thing is that it leaked and caused damage. Retro computing community thinks that the most likely cause is battery damage.

Here is the exact model from someone else: https://theretroweb.com/motherboards/s/msi-3121-v3

Battery (top-left) already removed, but it shows that this one has leaked before as well. When you look closely, you see battery residue on the nearby 8-bit ISA (?), so it must have leaked a lot at some point and been cleaned up. Unfortunately, it came with a notorious Ni-Cd Battery; even for its time not the best.

I have an old mainboard from 1990 with emotional value and a leaked BIOS battery. And advice for cleaning that?

lol, getting all displays working is indeed my biggest worry for my last Windows PC, migrating next month. It has both an NVIDIA and a Radeon GPU, and that works great on Windows. But a quick test boot from USB did not go so well on Ubuntu, so the truth will only come out after a real install with drivers.