Info: Your cart items have changed. Please review your FedEx shipping rates.

[beetlejuice]

Been receiving this error when trying to complete checkout. I simply add one product to the cart and proceed to the checkout. I select Fedex as my shipper and click save, I then get this error, but it's not like I've gone back and added or subtracted anything from the cart since the original item was added. It's just one item.

"Info: Your cart items have changed. Please review your FedEx shipping rates."

I have reinstalled the Fedex calculator and the error remains. Seems to have happened after upgrading to Version 2.0.18a but can't be 100% sure that's the cause. May be something more simplistic, but wasn't happening under the old 2.0.14 version.
I also checked it wasn't an issue with the product itself, so I've changed the product (after closing the browser and clearing the session) and the error still remains, the customer can't get past this to the payment page.

Does anyone have any ideas where to start?

Cheers