You are not able to log into smartCARS with the correct credentials, but you are able to log in via your website.
Make sure that there are no redirects in place on your website that affect the frame.php file in any way and that they are set to redirect POST data as well.
This problem can occur when a redirect, either via cPanel, .htaccess file, or CloudFlare (or similar services) is in place on your website. These redirects can lose the POST data that smartCARS sends to your website. That data includes password information you gave in your smartCARS client. When this happens, smartCARS receives no POST information so it shows that the password is incorrect.
If you do not have the above prerequisites, and you are still experiencing the issue, please open up a ticket.