Okay, more insights:
For the things dependent on the number of CPUs, I found a suspect:
I suspect this to be L9 or L10 from the schematics (EDIT: it's in fact more likely to be L2000, opening a different thread about that). I'll try to find someone who can help me replace that sucker. Before that, there's probably little sense in trying to resolve the other issue.
- The bookworm kernel 6.1.0-39 manages to enumerate the SATA card reliably if I limit the number of CPUs to 2.
- The trixie kernel 6.12-something reliably fails to boot with the SATA card in no matter the number of CPUs allowed.
- Without the SATA card, the trixie kernel boots cleanly with only two CPUs.
For the things dependent on the number of CPUs, I found a suspect:
I suspect this to be L9 or L10 from the schematics (EDIT: it's in fact more likely to be L2000, opening a different thread about that). I'll try to find someone who can help me replace that sucker. Before that, there's probably little sense in trying to resolve the other issue.

