@CodeMercenary Looking for in detail would need a support tunnel
it's probably a vdi that has been deeemed incorrect by the cleanup script. The easiest way to fix it in mirror backup is to delete the most recent backups of this VM on the target , they will be re transferred from the source.
If your retention is the same, you can even ourge all the backups of this VM on the target and restart the mirror backup
the mix shouldn't be an issue in itself