This error is reported during backups for sets using VSS and is generally due to another VSS-based backup solution or for instance scheduled windows shadow copies interrupting the DS-Client backup.
To Troubleshoot this issue:
1. Check that no Microsoft VSS operations are running during the scheduled Asigra backup times, including windows backup or shadow copies
2. From an elevated command prompt run VSSADMIN list providers to list any 3rd party VSS providers, if third party providers are present either hardware or software contact us for further instruction