PayPal Popup Error Window

Hello WPMU Dev Support,

The FLOUT.CO website has been set up to accept payments for products using PayPal Express gateway. I have been testing it as a customer and purchasing products, the following error window keeps popping up:

0) IE 9 is being used
1) Have put a product into the cart
2) Proceed to checkout
3) Enter my shipping details
4) Get to the PayPal window to pa for the purchase
5) Enter my PayPal Username/Password
6) An error window pops up
7) Click YES to continue and the purchase is completed as expected

Please see attached document for screen shot of pop-up window.

Also, on return from PayPal the message was not displaying the PayPal Transaction ID nor the amount spent. This occurred twice but has mysteriously now working, any hints as to why would be grateful.

Really cannot have this happening on payment, perhaps an issue with your API with PayPal.

Can you please help to resolve ASAP.

Kind regards,
Yusef