That appears to be the same error discussed in this other thread:
http://www.e-junkie.com/bb/topic/4708
Googling for the URL that was "not found" in that error, apparently it has something to do with your already being logged into a PayPal account (maybe just merchant or developer accounts?) when you try to test a checkout, so logging out of your PayPal accounts, or better yet clearing your browser cookies, should solve the problem.
If you could provide more details about exactly when in the checkout process this error appears, and whether you're using PayPal Standard or PayPal Pro, we could post that to PayPal's X.com developer forum to confirm whether the "already logged in" explanation is accurate and perhaps get further details.