Their solution was to create a new machine
and then to re-import the disk image from the defunct one into the new machine.
Seems a bit silly but seemed to work for them.
No idea what the reason was.
I also had one of my VM’s not working anymore
with the very same symptom -
but since it was for testing only and I wanted to use Bios instead of UEFI anyway
I just deleted it and created a new one - so I don’t know whether what I described will work.
It appears that the latest edk2-ovmf changed the way that firmware auto-selection functions. I was able to get a VM to boot by editing the XML file as described in the following Arch Linux bugtracker post.
Thanks for finding and sharing that!
It was a mystery to me, into which I did not want to invest too much time to solve, so I simply ditched the whole VM to which this happened.