Phuw, this took me quite a while to nail down.
I have a server running in the web hosting a socket.io connection behind a reverse-proxy for ssl accessable via a domain.
When I try to connect to the server via wss://my-domain.net I simply get nothing. No request to the server or anything!
At first I thought it was a problem with the reverse proxy, but connecting to the same address using the python-socketio module opens a connection no problem (and shows handsahe in the server logs).
The suspicion that it does not resolve the domain comes from the fact that A: It was working no problem in the localnetwork or when bypassing the reverse proxy via public IP and B: When opening the port to the server directly and setting the IP directly also works (but in this case without ssl [no go!] )