More importantly, what does df say is available? Is it supporting the idea that it ran out of space?
What does ls -lrt show passing through? Duplicati files typically have names starting with dup.
SQLite temporary files can apparently be invisible, so ls might not spot them, but I expect df will.
What sees space? I see nothing posted. How exactly does Duplicati claim, e.g. what messages?
Possibly you’ll need to look in About → Show log → Stored. A backup failure lacks its normal log.
If you’re changing any sizes away from default, please post the new values. Changes can matter.
If you have a different large folder that’s actually on disk, you could use tempdir to use it. Manual information appears to be a bit stale. Currently tempdir changes both Duplicati and SQLite uses.
But if you’ve got something misconfigured, it would be better to figure out what that is to correct it.