Do you run the server on your local machine? In this case it's important to keep the "server_ip" field in the server.properties blank, and also make sure that all ports are forwarded (by default, ports 4254-4259 TCP and UDP).
Can you send me the IP + port of the server (if you don't want to post it here, you can alternatively send it via PM to me)
Still no luck. I do run it on my machine.