Cause: The group name is null, the group name is misspelled, the invocation or given database is not the masterdef, or one of the masters does not believe the invocation database is the masterdef. See the solution below.
Error solution: If the given group name and masterdef were both correct, connect to the masterdef and retry the request, or relocate the masterdef at the (errant) databases using relocate_masterdef.
Database version: Oracle 11g Release 2 (11.2)