If you use Windows Server Backup or any other VSS aware backup utility you should make sure and pick up KB956697 – Windows Server 2008 Hyper-V VSS writer is not used during a backup job because of corrupted or invalid virtual machine configuration files. next time we should go for a shorter title…
The basic issue is that if you have a single invalid or corrupted configuration file none of the virtual machines will be backed up properly on that server. I have also seen this get triggered by anti-virus or other utilities so the long and short is if you backup your servers you should make sure you have this update.
Hyper-V Integration Test Lead