i have this problem too. i can solve the problem TEMPORARILY by clearing all cookies associated with paypal from my cache (use the search feature in the cookie dialogue and search for "paypal.com"). Unfortunately, this is only a temporary fix. Paypal will work normally for a little while, but the problem will always reoccur... sometimes a day later, sometimes a week later.

Yes. In such a case you need to remove the cookies from sites that have that problem. That shouldn't be necessary that often. Make sure that you do not have cleanup software running that may corrupt the cookies.sqlite file.