This could be an issue with Shadow Copy Components on the Server being backed up.
Resolution:
1. Usually, a reboot of the server will resolve this issue. If a reboot fails to resolve the issue, check the VSSADMIN writer status (follow the tech note given in the Related Documents section).
2. If the VSSADMIN writer shows failed status for any of the writers, refer to the Microsoft Article given below for the Volume Shadow Copy Service (VSS) update package.
http://support.microsoft.com/default.aspx?scid=kb;en-us;940349
For further help with writers, please contact Microsoft for assistance.
3. If the VSSADMIN writers shows a "stable" status, update the server with the Volume Shadow Copy Service (VSS) update package given above and test the backup again.
If the issue remains unresolved contact Symantec Technical Support for further help.