PowerProtect: Server DR replication configuration job failed

After clicking the 'Enable Replication' for PowerProtect Data Manager Disaster Recovery (DR) backup, the configuration job failed with error 'Unable to establish connection to Data Domain'.  
 

serverdr.log has below messages, or errors logged.

2022-08-29T05:56:37.455Z INFO [] [SimpleAsyncTaskExecutor-4] [][][][][] [c.e.b.s.common.SshManager.connect(121)] - *******timeout: socket is not established*******
2022-08-29T05:56:37.455Z INFO [] [SimpleAsyncTaskExecutor-4] [][][][][] [c.e.b.s.common.SshManager.sendCommand(143)] - Could not connect to the host <DD_hostname> with username: <DD_username> password: *****
...
2022-08-29T05:58:07.483Z INFO [] [SimpleAsyncTaskExecutor-4] [][][][][] [c.e.d.e.c.s.c.s.i.AutoConfigServiceImpl.configureStorageUnitForReplicationTarget(5748)] - configureStorageUnitForReplicationTarget exception: com.emc.dpsg.ecdm.cs.serverdr.core.exception.SdrException: SDR_REPL_SSH_NULL_ERROR code=500 reason="Unable to establish connection to Data Domain <DD_hostname>, received null response" remediation="none"

PowerProtect Data Manager cannot establish an SSH connection with either or both source and or replication Data Domain. 
 

To enable replication for the Server DR, PowerProtect uses SSH (port 22) to connect to replication source and target DD storage systems.
Port 22 for both source and target DD systems must be opened to the PowerProtect server.