Server ONLY works in Firefox

I grabbed an old project to get the network code from it.
It's a networking-test-project (nothing more here except a player-model and a level-model), so I added the whole thing.

I wanted to make a server application (run in background...), but it seems, that it only runs in Firefox.

I tried to make a server with firefox, google chrome, internet explorer, and standalone windows x86_64 application.

Did not work with 127.0.0.1, local IP and external IP (except firefox, where I only tested...

Server ONLY works in Firefox