GCP Google cloud App engine socket.io won't connect by wss - google-app-engine

Question

I'm using nodejs and run a very simple socketserver based on the examples of Google. I've basically set everything according to how they explain, my environment is flex etc. But yet everytime my application client (angular based) produces the following error in the developer console
WebSocket connection to 'wss://' failed: and falls back to https polling
It works perfectly fine when I run my application locally.
My socket app.js
'use strict';
const express = require('express');
const app = express();
const userSocketIdMap = new Map();
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.set('view engine', 'pug');
console.log("clienturl is:", process.env.clienturl);
const server = require('http').Server(app);
const io = require("socket.io")(server, {
cors: {
origin: process.env.clienturl, //this is filled with correct origin url
credentials: true,
},
allowEIO3: true
});
app.get('/', (req, res) => {
io.emit('message', { message: `GET call` });
res.status(200).send()
});
app.post('/', (req, res) => {
let message = ''
if (req.body.message.hasOwnProperty('data')) {
message = Buffer.from(req.body.message.data, 'base64').toString(
'utf-8'
);
}
let sender = '';
let uid = '';
if (req.body.message.hasOwnProperty('attributes')) {
if (req.body.message.attributes.hasOwnProperty('sender')) {
sender = req.body.message.attributes.sender;
}
if (req.body.message.attributes.hasOwnProperty('uid')) {
uid = req.body.message.attributes.uid;
}
}
if (uid) {
let recipientSocketIds = userSocketIdMap.get(uid);
if (recipientSocketIds) {
for (let socketId of recipientSocketIds) {
io.to(socketId).emit('message', { message: message, sender: sender, uid: uid });
}
}
} else {
io.emit('message', { message: message, sender: sender, uid: uid });
}
console.log({ message: message, sender: sender, uid: uid });
res.status(200).send()
});
io.on('connection', socket => {
let uid = socket.handshake.query.uid;
if (!userSocketIdMap.has(uid)) {
//when user is joining first time
userSocketIdMap.set(uid, new Set([socket.id]));
} else {
//user had already joined from one client and now joining using another
userSocketIdMap.delete(uid);
userSocketIdMap.set(uid, new Set([socket.id]));
}
console.log('client connected', process.env.clienturl,userSocketIdMap)
});
io.on('disconnect', socket => {
console.log('Got disconnect!');
});
if (module === require.main) {
const PORT = process.env.PORT || 8080;
server.listen(PORT, () => {
console.log(`App listening on port ${PORT}`);
console.log('Press Ctrl+C to quit.');
});
}
module.exports = server;
And my app.yaml file
runtime: nodejs
env: flex
manual_scaling:
instances: 1
network:
session_affinity: true