I am using Manjaro, using the same version as you. In my case, the following solved the problem.
The configuration was supposed to be in /root/.config/Duplicati/. However, the systemd file (which was damaged, it had the wrong user) pointed to a EnvironmentFile in /etc/default/duplicati. In there was the definition DUPLICATI_HOME=/var/lib/duplicati.
Following the path found that the structure was similar to /root/.config/Duplicati/ but some files were missing, specifically a database.
I copied all the files from /root/.config/Duplicati/ to /var/lib/duplicati/ (after making a backup of course).
After I fixed the service file, I reloaded the systemd unit and restarted the service. I used systemctl daemon-reload systemctl reastart duplicati
The backup jobs reappeared and they started, as they should.
Hope this helps you too. Please let me know what happened or if you find a different solution
It would be worth a look at ~duplicati/.config/Duplicati to see what it has (ideally your config, etc.).
The historical issue with Arch is that AUR liked to run as a duplicati user (secure but complex).
Welcome to the forum @NBoumakis and thanks for the help. There might be related info at AUR: