Also just a "FYI": I recently had a similar problem while testing in the sandbox. After combing this forum, somebody suggested checking in admin that the paypal email address was correct. In my case, I had the 'live' email address for the 'sandbox' switch. Silly me. Seems like this error message is a symptom with more than one cause, because, in my case, simply correcting the email address solved the problem. Hope this helps somebody else.