You have kind of improper value set in
$main_url of setup_options.php.
$main_url should be the full URL to your forum. Even on testing environment, it should start with http:// and then follow by the "localhost" or some internal domain name. In the case if the forum is installed on the live server, this should be the full URL containing the domain name and the forums folder name. This value should NOT end with a slash.
For example, in the case of miniBB forums, this value currently is:
$main_url = 'https://www.minibb.com/forums';
