paypal-digital-goods

I am using the following code to accept payments for digital goods: https://www.x.com/blogs/Nate/2011/01/07/digital-goods-with-express-checkout-in-php I have a sandbox account and I have used the API credentials from the sandbox account. Everything works fine until I press the "Pay with PayPal" button. At this point when I try to login to continue the simulated transaction I receive the following error: "We are unable to complete your request at this time. Please try again later. We apologize for the inconvenience." I have no idea whet I'm doing wrong, can somebody help? There are a number of