User home *apparently* scracthed after Big Sur update
The key point of the problem is that my user profile (fritz) is located on a separate encrypted volume Users, so its actually home path is /Volumes/Users/fritz. When I login after a reboot I use another account to enter into the system (administrator) that is regularly located on the root volume (/Users/administrator): then I enter the encryption password for the Users volume and at last I log into my main account.
This setup (which has been working for years) probably messed up the latest system update tool, because it basically recreated a new user fritz from scratch (when I logged in after the system update I was not only asked for enabling Siri - it always does it - but also other preferences, such as the light/dark theme, a thing that happens for users that have just been created). The path of the new bogus account was /Volumes/Users/fritz as it should be but it was not a mount point: it was just a plain directory of the root volume. The Users volume was mounted at /Volumes/Users 1 and everything was still there, but no more associated to the fritz account.
The obvious solution was to remove the bogus /Volume/Users directory, but the system wouldn't let me do that, even from sudo - typical security restrictions of the latest macOS's I suppose. I rebooted in Recovery mode just to find - to my surprise - that the bogus directory had not been created in that mode; so it was impossible to remove in that way.
At last the solution was:

Log in as administrator as usual, without entering the password for the Users volume.
Remove the user fritz.
At this point I was able to remove with sudo the bogus directory /Volume/Users.
Open the Disk Utility and mount Users. Now the volume was mounted in the right place.
Re-create the fritz user with all the previous settings (UID and home folder path).