ORA-12537: TNS:connection closed - standby error in DGMGRL

Error: ORA-12537: TNS:connection closed

Details: Occurred on a standby database after switchover from primary to standby. Error came up when checking DataGuard broker configuration.

Cause: The mrp process not running on standby
Solution:
Checked if MRP process was running on standby by executing:

ps -ef | grep mrp

Nothing was running.
Performing regular maintenance on standby database fixed the issue

SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP NOMOUNT;
SQL> ALTER DATABASE MOUNT STANDBY DATABASE;
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION;