on the checkout failure page, add the following code after the closing </html> tag:
<?php var_dump($_SESSION); ?>
I am going to start a new back end ticket to look into the issue with you.
I have created a support ticket so we can look into this issue further.
To view and edit your support ticket, please log into your support history:
supporthistory.php
If anyone else is experiencing this same issue, please append to this thread.