Vista - The VSS writer failed to get the properties of the virtual machine

Hi!

Last weekend, one of our Virtual Server 2005 servers repeatedly reported the
following event:

"The VSS writer failed to get the properties of the virtual machine <VM
name>, since the operation did not complete in maximum allowed time. The
virtual machine is allowed for Offline backup only. Check if the virtual
machine <VM name> is running and then retry the operation, if needed to be
backed Online."

Although we backup the VMs using Veritas NetBackup, we don't use VSS.
Instead, we shutdown the VMs and backup them. It always worked well, but this
weekend the vssrvc.exe process went to 100%, leaving the entire server
unresponsive, and a flood of the above event was written to the Event Log on
the same time that the backup should run.

Could anyone at least explain me what happened? I didn't find any kind of
information regarding the listed event.

The affected server is a Virtual Server 2005 R2 SP1 server, running ~ 25 VMs.

Thanks!
Eduardo