Backup unusable: Duplicati tells me to repair, but repairing doesn't work

[JonMikelV]

This sounds like what happened over here:

Problems with repair

This happens because the file in question appears to have no content, which should not happen (meaning, Duplicati should not create such a file). Does the file exist on S3? If yes, what size is it? What happens if you remove it from S3 (please keep a copy elsewhere)? If no, I think the problem is that Duplicati somehow “knows” that the file should be there even though it is not. When it then tries to figure out what contents are in that file, it fails (should be fixed in code then).

Did you check that the the 8 missing files for backup 2 are REALLY missing from the destination?

My guess is that there are some dindex files reference missing (or empty) dblock files and that moving those dindex files elsewhere (or deleting them, if you want to play it lose & wild) will let the list-broken-files and purge-broken-files commands work as expected.