And I should have guessed, that this is only how trouble starts =:-O
I immediately removed the locked snapshot image (actually they pretty much seemed to
disappear by themselves as delete operations might have been pending) but the VM that was
doing the snapshot, is still there, even if the disks are long gone.
But there are still references in the VM to two (now gone) snapshot images with a status
'illegal' (because they don't exist anymore) that I cannot delete from the
VM... without which I cannot delete the VM.
Any idea how to fix that?