Agreed. If this is a shared team drive, I lack one but can try citing sources. 403 Forbidden
The 403 (Forbidden) status code indicates that the server understood
the request but refuses to authorize it. A server that wishes to
make public why the request has been forbidden can describe that
reason in the response payload (if any).
Are you doing this through the web interface with the same login as Duplicati, or are the logins different?
Is Duplicati using your personal drive or a shared drive? If a shared, what is its access level to the drive?
If Duplicati has Contributor access, it would be able to upload, but delete needs Content manager or Manager or 403 seems the likely result. There are other ways to get a 403 (e.g. by excessive uploading), however, for a delete, a permission error seems more likely. How many backups happened before 403?
Resolve errors lists a lot of ways to get a 403. Possibly you could find some details in a message below the error itself (which is very generic). If About → Show log → Remote doesn’t get it, you might need to view live logs at About → Show log → Live to catch it, or set up a –log-file in order to see the later lines.
Logging would also show why the delete is being attempted. If you have run backups for awhile, it might have become time for a compact, however an upload retry (after a network error) will also delete first try.
You could view your job log (Show log under the job) at failure time, e.g. is CompactResults meaningful?
Retries of uploads can be best seen by a log a Retry level (if to a log file the setting is --log-file-log-level) but can be inferred from RetryAttempts in your job log, or (I think) from job Show log → Remote doing repeated put attempts of a file that’s the same size and hash, but whose name changes (it’s renamed).