The error "java.net.UnknownHostException: " suggests that the PRPC host is unable to resolve the hostname "pdn.pega.com" - rather than it being an issue with specific protocols (but I could be wrong - I am unable to explain why your CONNECT-HTTP rule is able to reach 'http://pdn.pega.com' ?)
From the PRPC Server's host Operating System : can you run:
And confirm that you get a DNS Record Response (as opposed to a '** server can't find xxx' type message).
Like John said, issue could not be specific to the type of protocols being used as secured/unsecured since if there is an issue with HTTP/HTTPS then the error should've come related to SSL something as 'peer not authenticated' etc.
Have you tried using IP address instead of domain name and try to connect to service?
I did, the problem was in our version of Pega itself (7.1.8), there was a problem with the REST connection that had been patched in 7.1.9 so a Pega support representative asked the engineering team to create a hotfix which was applied to our system which fixed the problem. I would suggest raising a formal SR with the information I have just mentioned so they can examine your logs and potentially create a patch for your platform also.
Thanks Jack. We got the issues resolved by pega folks.. They had an issue with Maintain session while doing https connection. It was actually creating new client each time even if maintain session on was on. We got the hotfix and is working as expected..