Weird server connection problems

So I was helping someone setting his server up. Everything went well, until it was done.

It starts up normally, I don’t see any errors. But when I try to connect via his WAN IP, I can’t find the server. If he joins via the same IP, he can join. Weirdest thing is: He can’t connect with localhost or 127.0.0.1.

Any ideas?

PS. Yes, ports are forwarded.