Wrong validation in IE and OPERA

please, take a look...

http://www.makey.by/ru/catalog/product/008-07-02-10

add this product to your cart (brown button in the middle)..

now, go to your cart

http://www.makey.by/cart

and press second brown button on the right (proceed).

now, you are on the page there you have to fill user info.

first, try to open this page with firefox and press brown button a the bottom of page.

the page is validated and errors messages are display.

same happens with chrome.

but with IE and OPERA, loading animation starts and never finish...

in drupal log, i have found 2 errors that i think are related.

Location http://www.makey.by/ru/system/ajax
Referrer http://www.makey.by/ru/checkout/275
Message Invalid form POST data.

and

Location http://www.makey.by/ru/system/ajax
Referrer http://www.makey.by/ru/checkout/275
Message Notice: Undefined index: form_build_id in ajax_get_form() (line 320 of /home/makeyby/public_html/includes/ajax.inc).

any advice?

thank you.