It may be something that the updated. I think the best solution is to purchase and install an SSL certificate.
When they put in the correct address did they still get the error page, or did they get the message on the success page? I've added the error message to the top of the failure page so you can see the cause from paypal's perspective. That was how I saw the shipping address mismatch error in my tests.
The other site has credential issues. Maybe they updated their password recently and the site hasn't been updated. You can update the login information on confirm.php lines 151-152