Thanks very much. I have been working on this all morning. After your message, there was a load of updates in 1.6, more than 65 modules, including PayPal. I was hoping, but it didn't fix the error.
You were directly on target, becuase it called up the order-confirmation.php. So I uploaded these from my PS 1.5, all the order... php files in the root. Now, when returning to my site via the link in PayPal site, it now goes to my site with a list of the customer's orders.
It's a work around, but done for now. Thanks