Why is my Drupal web form not working with IE?

Question

I have a web form (built by the Drupal web form module) on my site. After a user has completed the form and clicked submit button, it should redirect their browser to the web form node page (node/1028/done?sid=26).

But it just did not work with all IE browsers. There was no JavaScript error shown in IE 8, but in IE 6 and IE 7 a script error did pop up.

I have debugged the web form code, and IE appears to not execute the webform_client_form_submit() function at all.

I have run out of ideas now... I guess there is a client-side error preventing IE from executing the server-side code (sounds not logic).