The below warning may occur when backing up a domain controller:
"The program has failed to back up Registry: Cannot create a file when that file already exists"
The domain controllers C:\ may be out of space.
Investigate the drive space on both the backup location and the domain controller. If the drive is full, free up space and then try again.
The below warning may also be included in the backup session:
"The program has failed to back up COM+ Class Registration Database: There is not enough space on the disk"