Kibana 5 not working behind nginx

Question

I have setup ELK using docker (https://github.com/deviantony/docker-elk).

Then I added a subdomain to nginx with this config:

location / {
   auth_basic           "closed site";
   auth_basic_user_file /var/www/passwd;
   proxy_http_version 1.1;
   proxy_set_header Connection "Keep-Alive";
   proxy_set_header Proxy-Connection "Keep-Alive";
   proxy_pass http://localhost:5601;
}

When I now visit this subdomain I see that Kibana loads but then fails.

This is what my browser console shows:

When I visit Kibana directly using the IP of my server and the port 5601 it runs flawlessly. This only happens when being proxy_passed through nginx.