PayPal Confirmation ends up at empty cart

I'm using eCart6 (latest version) and DW CC2014. The checkout pages have been generated by the checkout wizard. No edits done to them except adding a session dump debug to a couple of the pages to see what's going on. The files have been uploaded to a live host. We're using PayPal Express Checkout.

If I add items to the cart and checkout, I'm taken to PayPal where I can pay by credit card or using my PayPal account.

If I pay by credit card all ends well.

But, in another test if I chose to pay by my PayPal account, after logging in and taken to PayPal's shopping cart, there is continue button and a written message that says, "You're almost done. You will confirm your payment on [NAME OF WEBSITE displayed here]. I click Continue and end up on cart.php with an empty cart. Huh?

I've tried it three times with the same result. I can't for the life of me find the source of the problem. These transactions are being tested on a real PayPal account since I never could get the sandbox option to work.

Any clues anyone?
Thanks in advance for any help received.