The Control Center displays the message "Other instances of db-backup must be running, exiting" when it fails to perform an unscheduled backup.
Following error messages are seen in the Control Center and in BrightmailLog.log:
Cannot backup the database on the local host.
ERROR: Other instances of db-backup must be running, exiting
This error indicates that a scheduled backup is already running in the background. Status of a scheduled backup can be checked by looking at the "Scheduled Tasks Status" page (Status > Scheduled Tasks > Backups). Please wait for the scheduled backup to finish before performing an unscheduled manual backup.