Ubuntu + Nginx + NodeJS: 502 Bad Gateway - node.js

Question

Currently I am trying to build a small nodejs API which should work on my server behind an already existing and working nginx setup.
nginx.conf:
server {
listen 80;
listen [::]:80;
server_name *.mydomain.com;
if ($host = www.mydomain.com) {
return 301 https://$host$request_uri;
}
if ($host = mydomain.com) {
return 301 https://$host$request_uri;
}
if ($host = hello.mydomain.com) {
return 301 https://$host$request_uri;
}
return 404;
}
server {
listen 443 ssl;
listen [::]:443 ssl;
server_name mydomain.com www.mydomain.com;
root /var/www/html;
index index.html;
ssl_certificate /etc/ssl/cert.pem;
ssl_certificate_key /etc/ssl/key.pem;
ssl_client_certificate /etc/ssl/cloudflare.crt;
ssl_verify_client on;
location / {
try_files $uri/index.html $uri.html $uri/ $uri =404;
}
}
server {
listen 443 ssl;
listen [::]:443 ssl;
server_name hello.mydomain.com;
root /var/www/hello;
ssl_certificate /etc/ssl/cert.pem;
ssl_certificate_key /etc/ssl/key.pem;
ssl_client_certificate /etc/ssl/cloudflare.crt;
ssl_verify_client on;
location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_set_header X-NginX-Proxy true;
proxy_pass http://127.0.0.1:3000$request_uri;
proxy_redirect off;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
}
}
index.js:
const express = require('express');
const app = express();
app.get("/", (request, response) => {
response.end("hello world");
});
app.listen(3000, () => console.log('listening'));
So i have mydomain.com and www.mydomain.com and they have nothing to do with nodejs and work fine.
The nodejs site lies behind hello.mydomain.com and just returns a 502: Bad Gateway error. When I am on my server (where everything lies) and just do:
curl localhost:3000
I get the right response. So the nodejs code works (I even approved it locally), but the nginx is not able to act as a proxy and "speak" with the local nodejs express.
Does anyone know why this does not work? I already searched through many tutorials, but I just cannot find the solution. :/