Executed as user: ggg\00DW_xxx_AGENT. DBCC execution completed. If DBCC printed error messages, contact your system administrator. [SQLSTATE 01000] (Message 2528)  DBCC execution completed. If DBCC printed error messages, contact your system administrator. [SQLSTATE 01000] (Message 2528)  DBCC execution completed. If DBCC printed error messages, contact your system administrator. [SQLSTATE 01000] (Message 2528)  DBCC execution completed. If DBCC printed error messages, contact your system administrator. [SQLSTATE 01000] (Message 2528)  Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed. [SQLSTATE 42000] (Error 3023).  The step failed.

  Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed. [SQLSTATE 42000] (Error 3023).  The step failed.

You are trying to do two conflicting operations on database. It seems when backup was going on you tried to perform shrink operation on database. Please try again after backup operation completes you would be able to do it.

I must tell you shrinking of both data and log file causes performance issue, if you have a job which does so immediately disable it.

Please see answer given by me on this Stackexchange thread