Problem with IIS8 proxy

[Insanee]

Hi,

I've installed nodeBB and configured IIS8 with port 80 as proxy so I'm able to use
forums.{page}.eu for the nodeBB board.

The problem is, when I try to use the forum (e.g posting), this error appears: (it's in german)
Firefox kann keine Verbindung zu dem Server unter ws://forums.{page}.eu/socket.io/?EIO=3&transport=websocket aufbauen.
-> Firefox was not able to connect to the server at ws://forums.....