“Suddenly” without context is hard. Can you show some logs leading up to that? Verbose level might do, however Information level might also be enough. You also have a status bar near screen top with status.
Could not find the file…. /tmp is a similar problem undergoing troubleshooting right now. Historically, this problem seems to happen near the end of the backup. Network errors are also suspected. Need logs…
EDIT 1:
Connection dropout appears to corrupt database was one where a much larger backup than yours might overload something in Nextcloud and/or Duplicati on the list done at end of backup. Need logs to see…
EDIT 2:
The above issue gets into the possibility of doing unusually heavy logging efforts, if in fact the list failed.
Lacking logs, and because it sounds like you are just getting going (having no existing backups), it may be worth testing the latest Canary build v2.0.5.114-2.0.5.114_canary_2021-03-10 to see if its changes fix this. Sometimes Canary is very high risk, but this one should be very similar to the next Beta, hoped for soon…