System Console: configuration times out while saving

Hey all,

I have set up a fresh install of Mattermost 3.0.1 using some slightly modified Docker containers in Kubernetes. Websockets and the general UI are working, but XHR requests for saving the configuration time out.

There's a container running with Nginx as a proxy with the following configuration:

server {
   listen 443 ssl;
   server_name mychat.example.com;

   ssl_certificate /etc/nginx/certificates/mychat.example.com/fullchain.pem;
   ssl_certificate_key /etc/nginx/certificates/mychat.example.com/privatekey.pem;

   ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
   ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:ECDHE-ECDSA-DES-CBC3-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA:!DSS';
   ssl_prefer_server_ciphers on;
   ssl_ecdh_curve secp384r1;
   ssl_stapling on;
   ssl_stapling_verify on;

   ssl_dhparam /etc/nginx/certificates/dhparam.pem;

   location / {
       gzip off;
       proxy_set_header X-Forwarded-Ssl on;
       client_max_body_size 50M;
       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 $scheme;
       proxy_set_header X-Frame-Options SAMEORIGIN;
       proxy_pass http://mattermost;
   }

}

There's an upstream mattermost which points to the IP address and port where Mattermost is running.

There are no particular errors in on the server side, for example when I try to set the SMTP config:

[2016/05/17 20:16:17 UTC] [DEBG] /admin_console
[2016/05/17 20:16:19 UTC] [DEBG] /api/v3/users/initial_load
[2016/05/17 20:16:19 UTC] [DEBG] /api/v3/admin/config
[2016/05/17 20:16:20 UTC] [DEBG] /api/v3/users/websocket
[2016/05/17 20:16:20 UTC] [DEBG] /api/v3/teams/all
[2016/05/17 20:16:20 UTC] [DEBG] /api/v3/admin/analytics/standard
[2016/05/17 20:16:20 UTC] [DEBG] /api/v3/admin/analytics/user_counts_with_posts_day
[2016/05/17 20:16:20 UTC] [DEBG] /api/v3/admin/analytics/post_counts_day
[2016/05/17 20:17:16 UTC] [DEBG] /api/v3/admin/save_config
[2016/05/17 20:17:51 UTC] [DEBG] /admin_console
[2016/05/17 20:17:53 UTC] [DEBG] /api/v3/users/initial_load
[2016/05/17 20:17:54 UTC] [DEBG] /api/v3/admin/config
[2016/05/17 20:17:54 UTC] [DEBG] /api/v3/users/websocket
[2016/05/17 20:17:54 UTC] [DEBG] /api/v3/teams/all
[2016/05/17 20:17:54 UTC] [DEBG] /api/v3/admin/analytics/standard
[2016/05/17 20:17:54 UTC] [DEBG] /api/v3/admin/analytics/post_counts_day
[2016/05/17 20:17:54 UTC] [DEBG] /api/v3/admin/analytics/user_counts_with_posts_day
[2016/05/17 20:18:20 UTC] [DEBG] /api/v3/admin/save_config

The config.json is writable for everybody and the actual error I see in the UI is a timeout (We received an unexpected status code from the server. (504))

Any idea where to look for solving this issue?