Any pattern before? Duplicati checks for missing files at start of a backup, so issue might be on prior one.
You can look at prior results using both the usual job logs and the server log About → Show log → Stored.
Anything other than a clean run might have left an issue. You can also look for patterns in your Repair logs.
The one you show looks like it was replacing some surprisingly small dindex files, or are they all that size? You probably have some GUI or other tool to look in SharePoint. Also, is it always dindex files that are lost?
It would help to know the history of the missing files. Your database has this, if you’re willing to open to look, or post a sanitized version with Create bug report, or run a long-term log-file=<path> log-file-log-level=retry.
On that last note, looking in the Complete log at RetryAttempts might be useful, but they are not visible until they turn into an error after number-of-retries exhausts, and I already asked about issues in prior runs.