I was having a 500 Internal Server Error…
nginx logs were showing me the following
2016/03/17 12:09:32 [crit] 31488#0: *11192 open() "/var/lib/nginx/tmp/client_body/0000000005" failed (13: Permission denied), client: 84.91.XXX.XXX, server: host.com, request: "POST /trades_edit/edit_trade HTTP/1.1", host: "host.com", referrer: "https://host.com/trades_edit/125"
Solution?
chown -R www-data:www-data /var/lib/nginx