I repeated the update process with a reloaded system backup [Clonezilla-live 6.2.1-25] from 2019/07/07 which resulted in an identical error [login failure at startup] - and rapid key repeat on keyboard entry.
I backed up the faulty system [Clonezilla] before overwriting it [24Gb, can send if you tell me how]
When I backup up the faulty system, Clonezilla backup ended the session with an error message - something about 'update micro code'.
A few weeks back the Ubuntu "software update" had a micro code update which caused problems with my system immediately, so I reloaded a system backup from the week before and when it updated there was no micro code update given. Maybe this is my problem area.
After the reload of the Ubuntu system backup with Clonezilli from 2019/07/07, this error message appeared in Clonezilla:
Failed to execute shutdown binary.
Broadcast message from systemd - journald@debian - [Wed 2019-07-25 UTC:4:41:51 C11:
Systemd freezing execution
The errors may be related.
Help please
OK, hit CTRL-ALT F1, which will put you in the text-only console.
rm .Xauthority
OR
mv .Xauthority .Xauthority1_
Or whatever name you choose, of course.
Reboot, and log right in.