DGMGRL DataGuard Switchover "Error: ORA-16608: one or more databases have warnings"

Every year, we test out DR strategies. Recently during the Oracle Data Guard switchover test, I received the following error:

DGMGRL> switchover to  'APLSTY'
Performing switchover NOW, please wait...
Error: ORA-16608: one or more databases have warnings

Failed.
Unable to switchover, primary database is still "apl"

The dataguard log file - drcAPL.log shows the following on the primary database server.

SWITCHOVER TO APLSTY
Command SWITCHOVER TO APLSTY completed with error ORA-16608
04/27/2014 15:15:13
Site APLSTY returned ORA-16664.
Data Guard Broker Status Summary:
  Type                        Name                             Severity  Status
  Configuration               apl_primary                  Warning  ORA-16607
  Primary Database            APL                           Success  ORA-00000
  Physical Standby Database   APLSTY              Error  ORA-16664

Database version: 11.2.0.3 Enterprise Edition on HP UX Itanium Superdome (11.31).

Issue: The database logon trigger was causing the issue. The trigger prevents the connection from standby database during a switchover.

Solution: Disable the logon trigger in the primary and then perform a switchover.

ALTER TRIGGER SYS.NAMECHECK_BEFORE_DDL_DB_TRG disable;

Once the switchover is complete you can enable it by:

ALTER TRIGGER SYS.NAMECHECK_BEFORE_DDL_DB_TRG enable;

Here we use the default Oracle port 1521 for the databases. Other possible causes for the similar error include the non default port used in connect string of both primary and standby - tnsnames.ora and DataGuard broker configuration.

Reference:
Dataguard Broker switchover fails with an ORA-16664 and the standby DRC log shows an ORA-604. (Doc ID 1530881.1)
ORA-16664 ORA-16792 Data guard Broker (Doc ID 1228797.1)
Data Guard Broker reported ORA-16664 (Doc ID 1390892.1)
http://abhinavsarin.blogspot.com/2013/06/ora-16664-unable-to-receive-result-from.html