to get more details, go to the pp_checkout_failure.php page.
On the bindings panel, click the plus button and select eCart -> Checkout -> Local Checkout response.
from the gateway list, select PayPal Authorize Express Checkout.
Repeat this, and select PayPal Process Express Checkout.
this will add 2 new binding groups to the bindings panel.
Expand each one, add add the Full Request and Full Response bindings from both to your page.
this will show the information being sent to, and returned from the paypal system to help us see what is going wrong.
NOTE: the full response and full request will contain sensitive info that should be removed before posting here.