Paypal Error 431

Hi every one,
I have an installation of cs-cart 4.3.1, and from few months I'm getting message from customers that can't pay through paypal. After a little investigation I have found that customers get the HTTP error 431 when try to pay, and this happens only when there are a lot products in the cart, orders with few product are processed correctly.
I'm using the built-in PayPal payments of cs-cart (version1.0), withouth any customization on it.

Can someone plase give me some advise on how to troubleshoot this issue?

The fact that this problem happens only on big orders really makes me mad :mrgreen: