standby database opened for exclusive access

  Explanation:  Hot Standby replication has been started on the
  master database using exclusive access.  This occurs when the
  /NOONLINE qualifier is used, or the /ONLINE qualifier is not
  specified during replication startup.  When the standby database
  is in "exclusive" mode, user processes cannot attach to the
  database.

  User Action:  If exclusive access is not desired, Hot Standby
  replication must be terminated and restarted using the /ONLINE
  qualifier.