So for some reason checkout page functions (changing quantity and deleting product) return 404 error on the checkout page, the URL remains the same for checkout page. It seems to be some sort of ajax issue.
This started happening on last 3.5 version, so I upgraded everything WP and all other plugins. I disabled W3TC completely, default htaccess and even disabled Better WP Security, still 404 issue continues.
URL of product page so you can and see what happens:
Would really appreciate help.