We recently released KB956697 for Hyper-V.

This hotfix addresses a specific issue that we have identified with VSS support under Hyper-V.  The issue is that if you have any invalid / corrupt / missing virtual machine configuration files - VSS will silently fail to backup *any* of the virtual machines on your system.

With this hotfix installed and virtual machines with invalid / corrupt / missing virtual machine configuration files will still not be backed up - but any valid virtual machines on the system will be correctly backed up.

There are two scenarios where you might hit this problem:

1. You have some virtual machines stored on a network server, which is unavailable at the time you attempt to backup a server.
2. You have a Hyper-V cluster node that has sustained an unplanned power outage (this leaves invalid configuration files on the node in question - which are harmless except for the case of this issue).

Cheers,
Ben