Configure.php
From Zen Cart(tm) Wiki
Domain
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');