on the failure page, add the following code, this will write the session contents and include the response from paypal:
<?php var_dump($_SESSON); ?>
what version of eCart are you using?
Make sure you have the latest version installed, 6.0.5.
if you have a previous version installed, you should uninstall it from the extension manager before installing the newer one.
then delete the checkout pages and recreate them.