The NetVault: Backup log contains the following error:
Error creating volume snapshot(s): Unknown VSS error code 0x8000ffff
There will also be a matching Windows Event Viewer Application error:
Event Type: Error
Event Source: VSS
Event Category: None
Event ID: 8193
Volume Shadow Copy Service error: Unexpected error calling routine CoCreateInstance. hr = 0x80070422.
Additionally, the command vssadmin list writers returns no output.
The code 0x422 in the Event Viewer error indicates that the service cannot be started, either because it is disabled or because it has no enabled devices associated with it.
Make sure that the service "Volume Shadow Copy" is enabled on the backup client. After enabling the service, confirm that it is enabled by running the command vssadmin list writers.