How to securely connect to Nodejs backend server?

Hello,

I have been struggling 2 whole days already with this problem. I’ve set up a reactjs app, nodejs backend server and mysql database on the same droplet. I can visit the react site (www.xxx.com) without problems, my backend is also running. But I cannot connect frontend to backend, I always get “ERR_SSL_PROTOCOL_ERROR” (on my local windows machine, everything is working perfectly). It seems the backend server doesnt make a secure connection. When I do ip_address:8800?api-command, I get a response. So, it is working. I connect from FE to BE like this: “https://ip_address_droplet:8800”

I have to admit, I never used Nginx before, but this is my config file:

server {

        root /var/www/sitename.com/html/build;
        index index.html index.htm index.nginx-debian.html;
        server_name sitename.com;

        location / {
                try_files $uri $uri/ =404;
        }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/sitename.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/sitename.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}
server {
	server_name nodejs.sitename.com;
	location / {
        proxy_pass http://localhost:8800; #whatever port your app runs on
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }
}

server {
    if ($host = sitename.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


        listen 80 default_server;
        listen [::]:80 default_server;

        server_name sitename.com;
    return 404; # managed by Certbot


}

Can someone guide me into the right direction? What am I doing wrong?

Thx, Peter