SESSION variables not saving

For some time now, I've been using some code which I developed a while ago to store shopping basket items. 

This works using the PHP $_SESSION array.

Part of the checkout process takes the user to a third party site for payment and then returns them to the main website for order confirmation.

Upon returning to the main site, the $_SESSION array is read to recover various pieces of information set prior to the visitor being transferred to the payment site.

I've recently upgraded a site running CMSB v2.17 to v2.63 and in doing so, the above functionality has stopped working.  The $_SESSION array on return to the main site appears to be blank.

Can you shed any light onto what might be causing this?

I've now put the site back to v2.17 and it's working again but it would be good to understand what in v2.63 might be causing this so I can change my code to work with v2.63 if necessary.

Thanks, Paul.