check the DSx jumpers.

I think I did, I tried all four positions (0, 1, 2 and 3) and they only seem to respond to the system (but with the error message) when I set them to 1, like they originally came. Is this what you're talking about?

Yes, because different systems require different settings there. But does the spindle drive and stepper motor even start?
Maybe one is stuck and needs a careful push for a start after a long storage?

They both make a sound and the leds light up as they are trying to read the disk, but immediately I get the error. I don't think it's a physical issue since the 3.5 drive does the same thing when either 5.25 drive is present (it works fine, even with the same cable, if it's on its own).

It's such a pity! They look pristine 🙁

And the connection is correct?
Controller on the long end.
Drive B: usually 5.25" in the middle, no wire rotation between controller and drive B: connector.
Drive A: usually 3.5" at the end with wire rotation between controller/drive b: and drive A: connector.

Yes, they are connected exactly like that. If I swap any of the new 5.25 drives for the old one (a TEAC), this one and the 3.5 one will work, so a connection issue is unlikely.

Edit: There are two more sets of jumpers, though they can only be removed and not set in a different position. Maybe they are interfering with something?

If anyone is interested or having the same problem: I think it might be the motherboard. Now all of the sudden I can't get the 3.5" drive to respond either. It's a SOYO 5EHM 1.0, I'll check if I accidentaly moved a dip switch or something like that.

As for the 5.25" drives, I'll put together another system with all the scrap parts I have to test them.