Right after posting this, i have got the solution.
I had the idea of missing settings in my nginx config - which caused the problem.
Here is the relevant part of my reverse proxy config now:
location / {
proxy_pass http://127.0.0.1:8001/;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto https;
proxy_set_header X-Nginx-Proxy true;
proxy_redirect off;
client_max_body_size 4G;
}
https://community.xibo.org.uk/t/walkthrough-for-creating-reverse-proxy-with-nginx/23139