Failed to unfreeze the VSS writers because the snapshot time exceeded the 10 second timeout limitation.
Microsoft VSS has a 10 second timeout limitation between freeze and thaw events for VSS writers on a VM. When the snapshot time exceeds the 10 second timeout limit, the VSS writers return to a 'failed' state. This failed state occurs because the VSS provider did not thaw the VSS writers within the allowed time.
Processing stops.
Check the event log. Ensure that the writers on the guest VM are in 'stable' state before trying an application protection VM backup operation again. A snapshot operation must be completed within 10 seconds.