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.