Thank you all for the effort!
After recreating the docker container, I can see my configs again. Really weird!
One thing you can maybe confirm is my guess that the ID# are the ten-random-letter backup filenames.
Yes, I just didn’t want to share too much 
I suppose you could
grep --text \<a-backup-sqlite-name>on the DB to see if you can find it there.
Unfortunately, the command returns empty. Don’t know what I did wrong.
However, the
backup IDx ...sqlitefiles indicates that you have been able to run the updated Duplicati at some point.
The weird part is that now I have 4 backup database files:
.docker/duplicati/data/Duplicati:
total 4.3M
-rw------- 1 root root 66K May 3 02:00 'backup MFAMIQGHEG 20210504020000.sqlite'
-rw------- 1 root root 445K May 2 23:00 'backup MILNJBGBOD 20210503110000.sqlite'
-rw------- 1 root root 1.2M May 3 01:00 'backup OQVEHHMFDP 20210504010000.sqlite'
-rw------- 1 root root 103K May 3 00:00 'backup XXXFDBUMNR 20210504120000.sqlite'
drwxr-xr-x 2 root root 4.0K Apr 30 13:48 control_dir_v2
-rw------- 1 root root 34K May 4 02:00 Duplicati-server.sqlite
-rw------- 1 root root 82K May 4 02:00 MFAMIQGHEG.sqlite
-rw------- 1 root root 776K May 3 23:01 MILNJBGBOD.sqlite
-rw------- 1 root root 1.5M May 4 01:00 OQVEHHMFDP.sqlite
-rw------- 1 root root 122K May 4 00:00 XXXFDBUMNR.sqlite
Another info that might be important for discovering what happened is that I run the https://containrrr.dev/watchtower/ container to auto-update my other containers. Which happened to be the one updating Duplicati.
I can see all my backups stored safely and up to date.
From my side, I consider this case closed. But I’m happy to provide more infos if you want to continue debugging this case to see what went wrong.