How do I diagnose a missing VDI error with Mirror backups

[CodeMercenary]

I have a couple Delta backups that target the same remote. Yesterday I set up a Mirror backup between that remote and another remote. When I ran the Mirror backup yesterday, it was successful.

Today 7 of the VMs succeed in the backup but 5 of them fail with:
Missing vdi <guid> which is a base for a delta

Could this be caused by the fact that the target for the Mirror backup used to be a target for the Delta backup so it has files on it already. Do I need to purge the folder entirely? I thought maybe that would give it a head start and the reason I'm using the Mirror backup is because the Delta backup used to fail randomly on random VMs when backing up to both remotes.

How would I go about diagnosing this? I assume since the guid for the vdi is missing, it's going to be hard for me to find since I can't find something that's missing. Alternatively, maybe it's because it's currently a weird mix of old Delta backups and new Mirror backups and maybe over the next few days it will purge enough old stuff that it will start working cleanly.

Should I delete the entire remote target of the Mirror to get it started cleanly? Or, more likely, delete all backups on that Mirror for the 5 VMs that failed with this error.