For some reason (*) my /lib was suddenly empty.
Very nasty.
Long story short, here's how I fixed it:
* boot a live distro (of similar OS version/flavor)
* In my case: Xubuntu 20.04.6 (live) over 20.04.5 (=compatible, but slight library/version deviation)
* Mount the broken OS system disk.
* rsync -avP /lib/ /where/ever/the/broken/OS/is/mounted/lib/
* Unmount and reboot live distro.