Are you getting that error when you click a payment button and are sent to PayPal?
I use Google Chrome and get a similar error sometimes, when I am doing a lot of PayPal development work. For some reason the PayPal cookies get gummed up inside of my browser. Usually, I can simply clear my browser''s cache, cookies and recent browsing history and the issue is resolved.
As a side note, the Nice PayPal Button plugin does not utilize any cookies.
~ All the best