Nextcloud 23 upgrade result - No shared files

We had Nextcloud 21 on our server. Due to the alert messages in the software update module, I installed the upgrade to version 22, and restarted the server. More alerts, so I then upgraded to version 23.

The upgrade went terribly wrong. Nextcloud was left asking for the PostgreSQL database location and credentials. I rebooted the server again. Now Nextcloud would come up with a system error message and nothing else.

I uninstalled Nextcloud from the command line and re-installed. It now came up ok, and users could log in. Some users had data files stored inside the Nextcloud space, so I restored them all from backup.

The problem now is that all users have lost the sharing of their files. There’s simply nothing shared with anyone.

Is there some way that this could be restored? I don’t know where they are stored. I have noticed, by-the-way that the previous installation was using Maria DB, and now it’s been changed to PostgreSQL. My guess seems to be that the upgrade process failed to migrate the data from the previous database., causing the initial Nextcloud screen asking for the db location and credentials.