error 10413 - The totals of the cart item amounts do not match order amounts.

Hi Guru's
What's the cause of the following error:

10413 - The totals of the cart item amounts do not match order amounts.

I set up the free paypal gateway and the error comes up when I try to pay through paypal. When I try through manual payment the error doesn't show up.

Can you give me suggestions for what to look at? Everything seems to be set up correctly?

Thanks kindly.