" Executed as user: Domain\SQLServiceAgent. DBCC execution completed. If DBCC printed error messages, contact your system administrator. [SQLSTATE 01000] (Message 2528) Backup and file manipulation operations (such as ALTER DATABASE ADD FILE) on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed. "
The error message is pretty self explanatory. You have to wait for the first ALTER DATABASE statement to complete before executing another. In our case a scheduled job tried to execute at the same time as currently running one and caused this, after the former took an extraordinaryly long time to complete.