From Zen Cart(tm) Wiki
Let's assume your domain is localhost, for both http and https. You must not leave a trailing slash after the domain name, i.e. localhost/, which is wrong and will lead to trouble.
This is how it should look (assuming your URL is localhost):
// HTTP_SERVER is your Main webserver: eg, http://www.yourdomain.com // HTTPS_SERVER is your Secure webserver: eg, https://www.yourdomain.com define('HTTP_SERVER', 'http://localhost'); define('HTTPS_SERVER', 'https://localhost');