You should check the full request and response to see some more detail about the error. To get this info add the following code into you shipping_failure page where you would like to see the error messages:
<?php
//WA eCart UPS Shipping Include
require_once("WA_eCart/WA_eCart_FedEx_PHP.php");
?>
The full FedEx request is:<?php echo $_SESSION["WA_Cart_Coupons_FedEx_FullRequest"]; ?><br/><br/>
The full FedEx response is: <?php echo $_SESSION["WA_Cart_Coupons_FedEx_FullResponse"]; ?>