Lubuntu update caused big problems (help needed)

Thank you I will try that

In the meantime I tried another system-update, setiing `base-files`
 and `snapd` on hold first.

After a reboot and a cold-start I can confirm it went well and there 
are complaints whatsoever.
So it seems to be **very** certain that either of the two
 aforementioned packages **has to be** the cuplrit.

Well done. I think you should wait a bit longer now, see if any bug info about base-files or snapd appears.
Then if nothing appears, backup and try updating base-files

Regards
Neville