Hyper-V backup failure: Error while registering disk for backup

I'm using the Free edition of Vembu to backup a Hyper-V server. I'm getting success on 3 out of the 5 VM's that I'm backing up. 2 of them are giving me the above error message "Error while registering disk for backup". I've not been able to find any info about this error message in the community.

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