Backups failing with the following error, or even similar error messages pointing to VM files:
"The file "D:\exchange-vm\9D543816-28ED-4D5C-A496-947A73AED573.xml" is configured as a file pertaining to this VM, but seems to be missing. (Error code 'ALLOWED_012')
Please make sure that this file has not been moved and/or security permissions to this location are configured correctly. Alternatively, remove the file from the VM settings in Hyper-V manager."
This occurs due to the fact that the shadow copy that VM Backup is reading from gets dismounted, which can occur for a number of reasons.
The reason for the backup failing is due to the shadow copy getting dismounted. The reason will be listed in a "volsnap" source event in the Windows System event log.
You can take a look at the following article in order to see how to deal with "volsnap" errors.