I went through a quick test of this and can see the pp_confirm page but I did not attempt to confirm the transaction. When you get this error are you on the pp_checkout_failure page?
If so add the following code to the pp_checkout_failure.php page or checkout_failure.php before the </body> tag:
<?php
if(!session_id()) session_start();
echo("Auth request: ".isset($_SESSION['ECO_Auth_Request'])?$_SESSION['ECO_Auth_Request']:""."<br /><br />");
echo("Auth Result: ".isset($_SESSION['"ECO_Auth_Result'])?$_SESSION['ECO_Auth_Result']:""."<br /><br />");
echo("Get Request: ".isset($_SESSION['ECO_Get_Request'])?$_SESSION['ECO_Get_Request']:""."<br /><br />");
echo("Get Result: ".isset($_SESSION['ECO_Get_Result'])?$_SESSION['"ECO_Get_Result']:""."<br /><br />");
echo("Process request: ".isset($_SESSION['"ECO_Process_Request'])?$_SESSION['ECO_Process_Request']:""."<br /><br />");
echo("Process Result: ".isset($_SESSION['ECO_Process_Result'])?$_SESSION['ECO_Process_Result']:""."<br /><br />");
?>
Commenting out the credit card options should not have any effect on using the Express Checkout. When you add this code to your failure page it should let you know what the problem is with the transaction.