(this is just a FYI) on a side note: If we are talking about server 2008r2 then to back up the system state there is a bug where it doesn't work unless you assign the system partician a driver letter. I now assign them all letter "Q" (no real reason) and they all backup flawlessly.
I don't understand why it works that way, but it does.
Also you can try the VSS server writer roll up http://support.microsoft.com/kb/940349
What backup software do you use? If it's Backup Exec have you tried to fully update B/E and reboot.
Also, I think once i had to fix it rebuilding the WMI database
Try backing up WMI
as described in http://technet2.microsoft.com/windowsserver/en/library/872a660b-391b-486c-a0a8-daa6acfa0aaa1033.mspx?mfr=true - does this work?
Try rebuilding the WMI
database as per part 3 of http://seer.entsupport.symantec.com/docs/276742.htm