sebastian-brandt I'm aware that Deja Dup is in the Solus repos, but for reason(s) I can't quite recall I've been using the flatpak version for the last year or so and haven't had any issues. If you're curious to give that a try you can install the flatpak version here: https://flathub.org/apps/details/org.gnome.DejaDup
If you fancy trying a Gnome style backup application based off the borg protocol, I'd like to throw in the excellent Pika Backup tool which is regularly updated by the dev which I actually have started to replace my Deja Backup for most things. Check it out here: https://flathub.org/apps/details/org.gnome.World.PikaBackup
It doesn't have automatic back up scheduler or encrypted backups yet, so for those that's why I still hang on to Deja Dup, but if those aren't deal breakers for you, you could definitely give Pika Backup a try, I highly recommend it for something in the vein of simplicity like Deja Dup. Apologizes I wasn't able to help you with your initial error, I know this answer does little toward that, but hope it helps nonetheless!