Here is a quick summary of some error messages the PubNub Dashboard can emit for APNS push certificate verification.
Error message and likely cause/solution:
Error connecting to gateway.push.apple.com: Socket connection closed.
You uploaded a sandbox (development) certificate but you did not select sandbox checkbox.
Error connecting to gateway.sandbox.push.apple.com: Socket connection closed.
You uploaded a production certificate but you did select sandbox checkbox. Also, make sure that the person that generates the pem file is doing so in the appropriate iTunes Connect account.
... EOF occurred in violation of protocol
You most likely need to regenerate your push certificate.
... SSL routines:SSL3_READ_BYTES:sslv3 alert certificate revoked
You most likely need to regenerate your push certificate.
... SSL routines:SSL3_READ_BYTES:sslv3 alert certificate expired
You most likely need to regenerate your push certificate.
If these actions do not solve the problem for you or you encounter other errors when uploading your push certificate to your PubNub Dashboard account, please send details to support@pubnub.com and attach your .pem file.