Error pop-up in shopping basket

Hey guys,

When I click the "red cross" to remove an item from my cart a pop-up saying "error" shows (but the product still gets removed from the cart). The same also happens when I click "checkout" from the shopping cart (and it still takes me to the checkout).

This error pop up is nothing more than an ugly inconvenience and I'd like to know how to get rid of it if possible? And why it's showing up?

http://powderandplot.com

If someone could re-create the error pop-up and help me I'd be a very happy chappy.

Kind regards,
Adam

I just tried and there was no error.

DL