Hi I have just updated from the 1.56 presta to the most recent one 1.66 as the issue was happening then, I hoped the updated would solve it. it hasn't Initially the cart would not add but I have the PHP updated on the hosting and it has improved it a little... with it being the add to cart and talking to paypal I think it is the cart block that has an issue.
paypal version is 3.6.8