Registering a JON agent with its servers requires two-way communication between agent and servers. The JON Agent needs access to port 7080 on all JON servers, except in the case of SSL where port 7443 is used. Each JON server must be able to access each of the connected agents on a unique host and port pairing. The agent port is usually 16163.
If there are multiple clustered JON servers, make sure each agent can communicate with all servers in the JON cluster via the IP and hostname pairs as configured through the JON server administration console. The JON server used by the agent to register may not be the server it tries to use after initialization.