We have a Virtual Machine with Windows Server 2008 R2 that has VHDs mounted. Since we had mounted the VHDs, Windows Server Backup on the Server 2008 R2 Hyper-V host always failed. When we detached the VHDs again, the backup would run just fine. Unfortunately, Windows Server Backup once again is not able to produce a helpful error message. The only indication in the logs is the following message that at least points you to the right VM:
The number of volumes reverted does not match the number of volumes in the snapshot set for virtual machine 'ourvm' (Virtual machine ID 123456789).
We decided to unmount the VHDs prior to the backup and then mount them again afterwards.