Socket.io + nginx + SSL = *31 upstream prematurely closed connection - node.js

Question

I am trying to run a Node.js server (with socket.io) as a pub/sub server for my main app. I pushed it to the server and created a subdomain (with SSL). My client HTML page can load the socket.io/socket.io.js, but the WSS handshake dont work as expected:
WebSocket connection to
'wss://ws.example.com/socket.io/?EIO=3&transport=websocket&sid=ogEegJXhjFh5lplgAAAF'
failed: Error during WebSocket handshake: Unexpected response code: 502
I've got a SSL subdomain defined in my nginx like this:
server {
listen 80;
listen 443 default ssl;
server_name ws.example.com;
# ssl on;
ssl_certificate /etc/nginx/certificates/certificate-ws-example-com.crt;
ssl_certificate_key /etc/nginx/certificates/ws.example.com.key;
# Redirect all non-SSL traffic to SSL.
if ($ssl_protocol = "") {
rewrite ^ https://$host$request_uri? permanent;
}
location / {
proxy_pass http://localhost:6969;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection upgrade;
proxy_set_header Host $host;
access_log /var/log/nginx/example_production.access.log;
error_log /var/log/nginx/example_production.error.log;
}
}
And my Node.js (pm2) server responds me:
[error] 29726#0: *31 upstream prematurely closed connection
while reading response header from upstream,
client: 86.228.47.218,
server: ws.example.com,
request:
"GET /socket.io/?EIO=3&transport=websocket&sid=ogEegJXhjFh5lplgAAAF HTTP/1.1",
upstream:
"http://127.0.0.1:6969/socket.io/?EIO=3&transport=websocket&sid=ogEegJXhjFh5lplgAAAF",
host: "ws.example.com"