Updated to master & works like a charm ;).
v6 long polling doesn't work anymore
The long polling just don't work anymore. NodeBB is now trying the whole time to connect to the websockets and just don't try to fallback to longpolling.
From Chrome Console:
(195) WebSocket connection to 'wss://localhost.cf-simulation-proxy/socket.io/?EIO=3&transport=websocket' failed: Error during WebSocket handshake: Unexpected response code: 400
None, NodeBB isn't even reached because the browser tries the whole time to connect per websocket connection and do not fall back.
Resolved in https://github.com/NodeBB/NodeBB/issues/2554