A peculiar kind of error occurs whenever a merge operation is taking place and at the same time a VM backup is started:
VM <GUID> cannot be backed up, error -2147212301 (VSS WRITER ERROR RETRYABLE), The virtual machine '<VM Name>' cannot be backed up because either the file groups reported during OnIdentify has changed, or the supported backup type (hot backup or saved state backup) reported during OnIdentify has changed. Try not to perform any configuration changes to the virtual machine between OnIdentify and OnPrepareBackup. (Virtual machine ID <GUID>)
The reason for the error is Hyper-V can’t handle the backup request while a checkpoint is being merged to a running VM.
You’ll need to wait until the merge is completed and then run the backup again.
The merge operation status shows up in the Hyper-V Manager “Status” column next to the VM name to the right.
In addition, Hyper-V logs all these operations in the Event Viewer, look into the logs by navigating through the tree on the left:
Applications and Services Logs -> Microsoft -> Windows -> Hyper-V XXXXXXX
See bottom half of this article: http://backupchain.com/hyper-v-backup/Troubleshooting-First-Steps.html