Jenkins Slave Connection Timeout When Connecting
Found a solution, but it leaves at least one question.
To resolve this I altered the Jenkins global security settings to use a fixed port for TCP connections and made sure it was one of my enabled ports, connection goes through cleanly now.
That said - this should NOT have worked on its own. When trying to connect earlier the logs clearly stated that connection attempts at the given port were refused (exact same port, and it was enabled then as well.)
I can understand if the agent was trying to connect at a different port, but I don't understand why dedicating the port itself would make a difference to the connecting agent.