That error looks like some sort of glitch happened on PayPal's site; indeed, it looks like PayPal tried to show you a "500 Internal Server Error" but couldn't find that error page, so instead you got a "404 Not Found" -- i.e., you got the error page for the error page! o.O
Googling around for that not-found URL, I found some mention that it may have something to do with being logged into a PayPal Developer (www.x.com) account while testing a PayPal checkout in the same browser, so you might want to log out of any PayPal accounts you have and clear your browser cookies, then test your button again.