The Suhosin Extension
Suhosin is a PHP extension designed to improve the security of PHP installations. Although it's motives are good, sometimes it can wreak havoc with Form Tools installations. Suhosin lets you limit the max size of the GET and POST PHP variables, preventing you from having very large forms on your website.
You can find out if you have suhosin installed by looking at your php info file. For that, just upload a .php file to your server with the following content:
<?php phpinfo(); ?>