in the WA_eCart/PP_ECO_Scripts/PP_ECO_PHP.php file try changing the following code:
if ($useSB) {
$soapObj->soapURL = "https://api-3t.sandbox.paypal.com/2.0/";
}
to:
if ($useSB) {
$soapObj->soapURL = "https://api-3t.paypal.com/2.0/";
}