The BIOS says it was built 2018, and cascade lake was released 2019. So it would never be able to boot with 3204 without update.
Yes. Exactly. I didn't see that earlier but realized it when I used the 3104 and it booted perfectly.
I tried to upgrade the BIOS via IPMI. But I couldn't flash it somehow. Got always some errors.
But now I've run into another weired issue. After some time "an unknown sensor" initiated a forced shutdown. Looks like a heat issue as I come further with a cold machine than when I immediately restart. There is also a memory error. So I ordered again another RAM module. This time a DDR4 2133 as recommended earlier.
But the installation has to wait until after my France vacation (2 weeks).
Thanks a lot for all your great help and ideas. Will write an update as soon as I'm back.
@Sable: I do not plan to install Windows. But I will be able to give you a feedback on Ubuntu.