Nginx upstream sent too big header

Fix when Nginx is running in a proxy / reverse proxy mode

Edit your nginx.conf or virtual domain file:

$ sudo emacs /etc/nginx/sites-available/site.conf

Set the following in http or server or location contaxt:

server {
 proxy_busy_buffers_size   512k;
 proxy_buffers   4 512k;
 proxy_buffer_size   256k;
 # rest of nginx config #
}