You may run Commvault Backup Software or any other on a Hyper-V Cluster and your VSS Snapshots of the virtual machines fail from time to time.
At CommVault Virtual iDataAgent using Microsoft VSS and Hyper-V Installation, Backup and Restore problems they describe several reason for this type of failure. There recommend many missing Microsoft hotfixes (you should really install) and a disabled automount, but the top one issue - that happens most often from my expierence is - that you are just running out of disk space in one of the VMs located on the Cluster Shared Volume.