I had issues with this 1.5.0/2.4.0.5 update within nextcloudpi too: I could not login since all my nc users were gone and the database was nowhere to be found, but I was able to solve the issue by simply restoring the backup that was made during the update itself (belonging to the previous installed version, nextcloud 24.0.3.2).
The backup file was positioned in the /data/nextcloud/data/ncp-update-backups/ directory so I could not restore it from there, since a backup cannot be restored from within the /data/nextcloud dir. The solution I found was simple: copy the nextcloud-bkp_DATE_TIMESTAMP-VERSION.tar.gz in the main /data dir itself. So I typed:
cd /MY_DATA_VOLUME_PATH/nextcloud/data/ncp-update-backups/
cp nextcloud-bkp_DATE_TIMESTAMP-VERSION.tar.gz /MY_DATA_VOLUME_PATH
and then, in the ncp interface I went to nc-restore and it did restore my backup, while at the same time purging my whole nextcloud-hosted file structure! Ah ah, luckily I had backups and I can laugh about it now but it was a bit scary when I found out.
If you’re going to restore a backup made by nextcloudpi at the time of any update, please remember it does not include your hosted data, so please backup/move your whole data folder file structure beforehand to a safe place. It can be found in:
/YOUR_DATA_VOLUME/
Your files are in /YOUR_DATA_VOLUME/nextcloud/data/YOUR_USERNAME/files, but I think it’s not enough to backup them since I found that even my previous nextcloud backups were gone, and while they are not absolutely necessary I do regret not having them around anymore.
As @ZendaiOwl wrote to me before,
Take the data out of any directory defined as a volume in docker, those are scary easy to delete the contents of
and I can absolutely agree now! 
Ok, so what about now? Did the restore work?
Yes, after a full nextcloud hosted data restore (I had backups) I am now able to access my nextcloud just like before, it seems to work just like it did before that nefarious update.
I hope I didn’t make too many errors typing this, sorry for any mistake I did.