 <p>Usually when I have come across this it has been a fatal error in PHP somewhere. Have a look at your PHP-cgi log to see if it is in there. There should be something in the nginx log like this: <code>104: Connection reset by peer</code>. Depending on your setup this <em>(sorry, link dead)</em> might help but if you're using <a href="http://php-fpm.org/" rel="nofollow noreferrer">php-fpm</a> it won't.</p>