Thanks, so really the only differences are the alt mode messages. The failed to register partner alt modes (-5) error appears not critical as it shows up in multiple other threads that discuss displays still working with them (e.g. [1], [2]) and I think I had them before myself. This leaves the amdgpu driver message:
[ +0.143457] amdgpu 0000:c1:00.0: [drm] Alt mode has timed out after 214 ms
which is new and looks the most relevant because of [drm]. I don’t know how to diagnose this subsystem though.
Other random information: I did try to swap out the cards in slots 1, 2 and 4, i.e. the ones with display output, to no effect.