The Suhosin Extension
Suhosin is a PHP extension designed to improve the security of PHP installations. Although its 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: Then load that page in your browser and do a search for "suhosin". The settings that will cause the most problems are suhosin.get.max_vars and suhosin.post.max_vars. Anything under a few thousand will cause problems - if not with your forms, then with the Form Tools UI when trying to manage your forms.