OK. That seems to work. Thank you!
I edited this line:
DAEMON_OPTS="–server-datafolder=/mnt/4AF15A0435E762B4/Duplicati/"
in the file /etc/default/duplicati with this command in Terminal:
sudo nano /etc/default/duplicati
and restarted Ubuntu.
But the first time it didn’t work, because “–” (2 dash) had become “–” (1 dash)…
THEN I could login and Edit the configuration (turning on scheduled backups again) and under “Database” I had to change the database location too.
So again: Thank you!
PS: It’s a bit confusing that when you start a “repair” it says “Starting backup”…