"Things don't appear to be working at the moment." for PayPal Express Checkout

[clcitalia]

Hello, I wonder if anyone can shed some light on this:

1. I am implementing PayPal Express Checkout on our site

2. I have the credentials and I've implemented the button that brings up the PayPal payment pop-up on our site.

3. I am using the 'Server-Side REST' example

4. I'm able to call a script on my server which uses the PayPal SDK to create a payment and the paymentID gets returned correctly to the 'button' script.

5. But after the button opens the pop-up, all I receive is '"Things don't appear to be working at the moment."'. No error message, nothing.

This is the part of the code that obtains the paymentID from my server:

// Show the buyer a 'Pay Now' button in the checkout flow
            commit: true,

            // payment() is called when the button is clicked
            payment: function() {

            var data = {
                "orderId": <%= CurrentOrder.O_Id %>
            }

            // Set up a url on your server to create the payment
            var CREATE_URL = '/ShopCart/ShopCart.aspx/PayPalExpressCheckoutPaymentCreate';

            // Make a call to your server to set up the payment
                return paypal.request({
                    method: 'post',
                    url: CREATE_URL,
                    json: { "orderId" : <%= CurrentOrder.O_Id %> }
                }).then(function (res) {                                       
                    console.log(res.d);

                    return res.d;
                });
            },

This WORKS CORRECTLY, the paymentID is returned correctly.

However, this next bit of code never gets accessed (it includes another URL to a script that records the payment successfully on my server):

// onAuthorize() is called when the buyer approves the payment
            onAuthorize: function(data, actions) {

                // Set up a url on your server to execute the payment
                var EXECUTE_URL = '/ShopCart/ShopCart.aspx/PayPalExpressCheckoutPaymentExecute';

                console.log(EXECUTE_URL);
                console.log('paymentID: ' + data.paymentID);
                console.log('payerID: ' + data.payerID);

                // Set up the data you need to pass to your server
                var data = {
                    paymentID: data.paymentID,
                    payerID: data.payerID
                };

                // Make a call to your server to execute the payment
                return paypal.request.post(EXECUTE_URL, data)
                    .then(function (res) {
                        console.log(res);
                        window.alert('Payment Complete!');
                    });
            }

This doesn't work and the pop-up remains open with the error message that things aren't working at the moment.

Anyone have any clue?

Since the JavaScript opens and is able to call the first script, I don't think this should be a problem of my 'url' being on localhost and the PayPal servers not being able to call it, right?