Socket.io + node js behind AWS ALB not working properly

Question

I needed a proxy service where I can pass as a header the ip address of the upstream. What I have is:

• node js + socket.io server that works locally
• Nginx behind an AWS ALB(the ALB is working on HTTPS and the Nginx works on HTTP so that the ALB is used as an SSL terminator.

I have the following configuration for Nginx:

server {
    listen       80;

    error_log stderr;
    access_log /dev/stdout;
    underscores_in_headers on;

    location / {
        proxy_pass_request_headers on;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection $connection_upgrade;
        proxy_set_header Host $host;
        proxy_read_timeout 86400;
        if ($http_X_Pass_To = "") {
            add_header Access-Control-Allow-Origin *;
            add_header Access-Control-Allow-Headers 'X-Pass-To,User-Agent,Content-Type';
            return 200;
        }
        proxy_pass http://$http_X_Pass_To$request_uri;
    }

    location /health {
      return 200 "healthy\n";
    }
}

map $http_upgrade $connection_upgrade {
    default upgrade;
    ''      close;
}

It reads the X-Pass-To header and forwards the request to the specified target.

The problem is that when I run the client it is working but I get this strange error which is not shown on local development: WebSocket connection to 'wss://<my-domain>/socket.io/?EIO=4&transport=websocket&sid=XggNamuD4_kpe2-JAABE' failed:

Also, when I use the client I get a lot of xhr requests instead of one as it works locally. And in the network tab I have one finished ws connection that shows N/A messages.

I have absolutely no idea what the problem may be. I would be quite happy if some of you know better what may be the reason for this strange behaviour.