I visited your site and the register page does indeed have a recaptcha (which is what I believe you mean by "security question"). If I am wrong please correct me.
Please understand it will be hard for me to tell which fields are which on the register page due to the language being german.
Not an IE9-problem... a think problem...forgot to logout the facebook-connection...that way the security question doesn't show up...although it might be a good idea anyway.