Ghost 14 will not successfully run a scheduled backup

I get the following error with Ghost 14:

 

Error EC8F17B7: Cannot create recovery points for job: Drive Backup of DRIVECPP1 (C:\), disk2 (D:\). Error EC8F03EA: Cannot create a virtual volume image of the selected drive. Error E0020002: Device \\?\SymantecSnapshot1 cannot be opened. Error EBAB03F1: Error performing inpage operation.
Details: 0xE0020002
Source: Norton Ghost

 

It will usually work if I do a manual backup, but will always get this error on a scheduled backup.  I am running on Windows XP Pro SP3, and am backing up to a NAS drive with plenty of space on it.