When I tried to upload a set of images, I sometimes got the error 504 Gateway Error. This happened because Nginx didn't wait long enough and rejected the request due to the default timeout. To correct this, I added the following lines to the site configuration: /etc/nginx/sites-avalaible which have been taken from this stackoverflow.
So basically, you have to add these lines:
proxy_connect_timeout 300;
proxy_send_timeout 300;
proxy_read_timeout 300;
send_timeout 300;