oscmax having problem with session

[rashmi_brw]

URL of website : http://acefancydress.co.uk/
Technology usedhp, mysql
CMS used : osCMax

I am working on website what is hosted on apache web server. If we are accessing website in windows machine we can do checkout process easily but if you try to place order using mac machine at the last page of checkout it redirects to previous page (checkout_payment.php). If user refreshes the final checkout page before clicking on button checkout process can be completed easily.
Problem getting started before 2 days after configuring apache server for SEO based URL. https redirection done at the server end by server administrator.
Is this problem related to cache or anything else? why it can not trace tep_session_is_registered('payment') and redirect to payment page.