Thank you very much, my problem is solved now! Pure magic!
The problem is (or was) that the transaction_id was already correct (if you trust the error message from vgchange -ay). So I changed it to the “wrong” one, 8558. I guesst it’s not why the problem got solved.
vextend qubes_dom0/vm-pool --poolmetadatasize -L+128M
This gave me a syntax error, however this
lvextend qubes_dom0/vm-pool --poolmetadatasize +128M
worked.
Then I tried
lvconvert --repair qubes_dom0/vm-pool
and, after some waiting, it fixed the problem for me. I can boot and even the VMs can start. Booting however takes a bit more time for some reason with a slight delay at
Job is running for LVM event activation on device 253:0
I guess I will just backup the qubes and reinstall the OS, just in case.