Here is another suggestion. Did you use the same email address & passwords for both your Adobe ID and your Sony Reader Library account? If not, then sometimes that can cause an error message.
Also, I would try to purchase a book through the Sony Reader Library and see if you still get the same error message. You can purchase a "zero-dollar" book so it doesn't cost you anything to do a test case. Go this website to see a list of free books and then search for it in the Reader Store.
http://inkmesh.com/free-ebooks/?site=sony