Is this the typical error, seen on DP backup for several months, with prior backup success?
Prior backup matters because PreBackupVerify happens before backup, so checks prior.
Do you have any logs with the actual file names, so that history of missing can be checked?
In addition to job log, server log at About → Logs → Stored might have them.
Next step is
You can option a log-file, maybe at retry level so that the file uploading will be recorded.
Missing file names might be in About → Logs → Stored.
The job database usually has 30 days of history, but it’s easiest to search using DB browser.
DB bug report is sometimes also an option.