If an instance of NTBackup is still running in the background and it has an Enhanced Storage Engine (ESE) database open, like Exchange 2000 Server storage groups, the Site Replication service, or Active Directory, a second attempt to connect to the database will cause NTBackup to post:
Event Type: Error
Event Source: NTBackup
Event Category: None
Event ID: 8012
Description: The 'Microsoft Site Replication Service' returned 'Backup is already active' from a call to 'HrESEBackupSetup()' additional data '-'
To workaround this problem, use Task Manager to identify and stop any hung , not responding, instances of NTBackup.
The Backup log may help to determine the cause of the problem.