AJAX filter issues in Internet Explorer

We have created a AJAX block filter for our shop at: [login to view URL] It is the filter in the left menu (exposed form in block using AJAX). In FF, Chrome and Safari we can click on the filter checkbox and all AJAX functions will execute normally. In IE (7&8), a first click will not register, but the second click will perform the expected action >> [login to view URL] It might be a race condition, we are really looking forward to your ideas to fix the issue. [login to view URL]