IE 8 gets HTTP 404 going to http://inteapp.com/index.php?bft=register

I have the registration form on a blog at http://inteapp.com (I used the shortcode). I made no changes. Works on Chrome and Firefox but not on IE 8. When I click the Register button and IE 8 tries to go to http://inteapp.com/index.php?bft=register I get an HTTP 404.

I am using WP multisite.

When I add click in the URL area and press enter it works.

http://wordpress.org/extend/plugins/bft-autoresponder/