Paypal Express Checkout: Looping back to Step 1

[ShannonL]

I am running Zen Cart 1.3.7.1
Database Patch Level: 1.3.7
Hosted by Camelot Hosting

I have not touched anything on the site to cause this to break and Camelot has also made no changes to the server. I called Paypal and they said it was a problem with my shopping cart.

My last successful Paypal transaction was on July 9th. Since then, I've been getting emails from customers who cannot check out using Paypal Express Checkout on my site: http://www.puffypp.com

Everything works fine until you get to the Confirm button on Step 3. Clicking the confirm button takes you back to Step 1 (delivery info). Clicking through the process again only causes and endless loop. No order is ever submitted, although an account is created in Zen Cart.

I have tried uninstalling the Paypal Express Checkout module and reinstalling.

I have tried reinstalling the Paypal patch from last year and uninstalling Paypal Express Checkout module, then reinstalling.

I have received two different error messages via email:

10417 Transaction cannot complete. - The transaction cannot complete successfully. Instruct the customer to use an alternative payment method.

and

10001 Transaction failed due to internal error - Internal Error

Any help is much appreciated. Thank you!