oliveee,
I have the exact same enviroment and the exact same error. Were you able to get this resovled?
Using BE 2010 R3, trying to backup Hyperv 2012 Clustered VMs. Remote agents are installed on the Hyperv hosts and VMs. The backups ran fine prior to creating the cluster. Once the cluster was created the VMs do not show under the individual servers. They are listed under the Cluster node name. I'm selecting the VMs under the Cluster node in the selection list as required.
AOFO: Initialization failure on: "VRTSRV::\\HyperV-Cluster.xxx.local\Hyper-V?HA?VM\VM1". Advanced Open File Option used: Microsoft Volume Shadow Copy Service (VSS).
V-79-10000-11230 - VSS Snapshot error. Unable to get minimum quiet time for physical volumes. Reduce the activity on these volumes, or wait and run the backup when there is less activity on the volumes.
The following volumes are dependent on resource: "C:" "C:\CLUSTERSTORAGE\VOLUME1" .
I could upgrade to BE 2012, but from your post above, it didn't appear to resolve your issue.
Let me know if there is a solution.