Make sure you have <?php session_start(); ?> on top of the page.... maybe that is the issue.
Try to view the "full request" and "full response" if it is blank, maybe you are using the wrong response. This is the failure page for the express checkout process. The the express checkout process has three steps, make sure you are adding the error during the "authorize" step if you get the failure page before seeing the paypal page.
My guess is you may have added the wrong response.