After a customer makes a purchase in Paypal standard on my Zencart store, hosted in Godaddy, the return to merchants icon takes them to a page with only two blank fields (such as a username and password) and a button that says "authenticate." Funny thing is that the transaction shows up an Zencart and the customer is able to log on for the download. I sell an eBook online and the customer needs to get back to Zencart from Paypal to download it. Any help would be appreciated.

Using 1.3.8a