The 400 status code means the HTTP request that was sent to the web server has incorrect format.
The 401 status code indicates that the user trying to access the source has not been authenticated or has not been authenticated properly. This indicates that the consumer must provide qualifications to be able to view the secured source.
I stopped using the sandbox a long time ago. A pain to use because you need to be logged in, different emails and it did not seem to be very reliable.
Do what Cyber said, use a real second account. I have a separate PP account just for testing (and backup). Though I usually test with a .01 priced product.