My feeling is that you have an extra PayPal Express Checkout server behavior on your confirm page. I would like to see the page to check on this. Please post back and include these pages as attachments to your reply. You will need to add them to a zip archive before uploading them. Make sure to remove any sensitive details like the PayPal API details from the page before posting it. If you are not able to do this please let me know and I will open a private ticket for you.