in your other thread, i asked you to add code to the failure page that would show the contents of the session.
for testing checkout, use a clean browsing session. in other words close the browser completely and restart it so that sessions created by other pages in the site will not add confusion to debugging the checkout process, basically, since this form is saving to a session, it will also show on the failure page along with the paypal response.
your testing to many things in the same browsing session and causing confusion when trying to look at the troubleshooting information.
in web site testing, you should be constantly restarting the browser.