High Sierra APFS partition disappeared from Clover menu

[ddddddd7777777]

My HS partition disappeared from the boot options without me doing anything new on the system. Just between boots.
I tried to narrow the problem down by removing all other HDDs from the system and resetting CMOS but it didn't help.
I also tried to boot from my HS install stick in order to repair my installation but somehow it's getting stuck at a pretty early stage (as in the whole progress bar is still grey and nothing happens. Not sure how to start this in verbose mode or something.)
I guess that's not really related though?

Any hints as on how to proceed?

The HDD is getting recognized by the BIOS and I also did boot an old Sierra partition that I still had in my computer and there I can find the HS APFS disk in disk utilities. So I don't think the disk itself is broken or anything.

EDIT: now I realized "XHCI Hand-off" was disabled after the CMOS reset. That's why my USB stick didn't boot. Still no luck with finding my APFS partition though.

 

[ddddddd7777777]

One thing I noticed now that I tried to repair my High Sierra by re-installing it (which should keep all my files) is this: the Volume is quite full. Only 4/500GB are available and the installer needs more than that. But I can't access the APFS partition from any other of my systems so how else could I delete some stuff on there?