You cannot login to smartCARS with correct credentials, but you can login via your website.
Make sure that there are no redirects in place on your website that affect the frame.php file or that they are set to redirect POST data as well.
This problem can occur when a redirect, either via cPanel's redirect system, a .htaccess, or CloudFlare (or a CloudFlare-like service) is in place on your website. These redirects lose the POST data that smartCARS sends to your website; that data includes the password you specified in the client. When this happens, the smartCARS web script believes you've given no password and will not let you login.
Note: If you do not have any of the above problematic prerequisites and are still experiencing this issue, please contact our technical support department.