Here’s what fixed it for me: Link
Basically, you are running PHP via Fast-CGI and your file uploads are exceeding Fast-CGI’s default maximum request length. To fix, either put the following in your .htaccess file…
<IfModule mod_fcgid.c>
MaxRequestLen 15728640
</IfModule>
… or, if you have access to your Apache config, add the “MaxRequestLen 15728640” line to your fcgi configuration file. (Mine was at /etc/httpd/conf.d/fcgid.conf, but yours might be at /etc/apache2/mods-available/fcgid.conf)