uwebsockets

I am on GKE using Istio version 1.0.3 . I try to get my express.js with socket.io (and uws engine) backend working with websockets and had this backend running before on a 'non kubernetes server' with websockets without problems. When I simply enter the external_gke_ip as url I get my backend html page, so http works. But when my client-app makes socketio authentication calls from my client-app I get 503 errors in the browser console: WebSocket connection to 'ws://external_gke_ip/socket.io/?EIO=3&transport=websocket' failed: Error during WebSocket handshake: Unexpected response code: 503 And