ssl – Nginx fails to proxy to Node

I have a running Node app that is live and works fine on http and https.

I setup Nginx and it is also running fine, tested with an sshtunnel, and it is getting a correct response from static files (such as MyPath/index.html).

However, I am trying to get Nginx to work as a reverse-proxy for Node.
Because I want to make another app on my machine, and Nginx should sort the incoming requests for each app.

But there seems to be an issue with Nginx I cannot figure out. I suspect it is a config problem. When I try to reach my Node app, I always get an error page from my browser, saying that there is an SSL issue.

Nginx config

server {
        listen (::):4444 default_server;
        server_name localhost mysite.com www.mysite.com;    

        access_log /home/mysite/access-log;    

        location / {
            proxy_pass http://127.0.0.1:5555;
        }
}    

I tried changing http://127.0.0.1:5555 to https://127.0.0.1:6666 but that didn’t change anything.

Node app

const port = 5555;
const secureport = 6666;    

…

const privateKey = fs.readFileSync('PATHTOCERT');
const certificate = fs.readFileSync('PATHTOKEY');
const credentials = {key: privateKey, cert: certificate};    

…
I use an express app instance here, also configured CSP with helmet. But I don’t think that’s the problem, because I disabled helmet and that did not solve anything.
…

const httpServer = http.createServer(app);
const httpsServer = https.createServer(credentials, app);    

httpServer.listen(port);
httpsServer.listen(secureport);