Symptoms
Trying to configure the link between an internal K2 server and a newly installed K2 blackpearl for SP2013 app. Installed the newest version and when entering the K2 URL there is a "500 internal server error" message, and the page freezes.
Diagnoses
It looks like there is some kind of restriction in the firewall preventing the call.
Resolution
Running the Registration Wizard from a server in the local network let us finish the registration.
https://support.microsoft.com/en-us/kb/837865 which talks about it.