Hopefully someone can help here. I've set up Google Checkout on a site for someone and everything works perfectly apart from the re-direct back to the store after successful order process.

What happens is, the user can go through checkout with google checkout and place the order and make payment, when this is confirmed in google checkout they click to return back to the store. However, on their return they get this message:

"Whoops! Your session has expired.
If you were placing an order, please login and your shopping cart will be restored. You may then go back to the checkout and complete your final purchases.

If you had completed an order and wish to review it, or had a download and wish to retrieve it, please go to your My Account page to view your order."

In the admin system of Google Checkout the order has been placed with google, and the order appears in Zen Cart admin.

However, when the customer returns to the Zen Cart store, their cart is still active and they're prompted with the error message above.

The store has a dedicated SSL.

Anyone know what can be causing this?