Agent out-of-sync/Unable to initialize ssl connection

You might have observed that sometimes after successful installation of the agent, when agent tries to upload data to OMS, upload fails.
I observed this behavior particularly when the agent is un-installed and then re-installed. Sometimes due to this operations, OMS tend to block the agents and rejecting the connections for upload.

Error - 
#$AGENT_HOME/bin/emctl upload agent
Agent upload shows following errors -
Copyright (c) 1996, 2009 Oracle Corporation. All rights reserved.
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EMD upload error: uploadXMLFiles skipped :: OMS version not checked yet..

-- Check the trace file 
#vi $AGENT_HOME/sysman/log/emagent.trc

ERROR http: 6: Unable to initialize ssl connection with server, aborting connection attempt: ret -1
ERROR pingManager: nmepm_pingReposURL: Cannot connect to https://oem.localdomain:7799/em/upload/: retStatus=-1

OR

Did not receive valid response to ping "ERROR-Agent is blocked. Blocked reason is: Agent is out-of-sync with repository. This most likely means that the agent was reinstalled or recovered. Please contact an EM administrator to unblock the agent by performing an agent resync from the console. Please contact EM adminstrator to unblock the agent"

Cause:

1. Agent is re-installed or restored from a backup, on the same port number as before.
2. The <AGENT_HOME>/sysman/emd/agntstmp.txt is manually deleted for some reason.

Solution -

Navigate to Setup -> Agents -> Click on problematic Agent name

+ Choose the 'Unblock Agent' option and click Continue
+ In Agent home page click on "Resynchronize" Button on right hand top of the page.
Ensure that the Agent is up and running when attempting the Re synchronization.

Once the re-synchronization is completed successfully, the Agent should be able to communicate with the OMS.