Do you have a site_url System Setting or Context Setting (you shouldn't, unless you're using multiple front-end contexts).
If you do have multiple contexts, check the site_url Context setting for each context.
If that's not it, check the core/config/config.inc.php file and look at the $site_url and $url_scheme settings. They should look like this:
$url_scheme= $isSecureRequest ? 'https://' : 'http://';
define('MODX_URL_SCHEME', $url_scheme);
$site_url= $url_scheme . $http_host . MODX_BASE_URL;
define('MODX_SITE_URL', $site_url);
Also, make sure you have this tag in the head section of all templates:
<base href="[[++site_url]]" />
If you have multiple contexts, it should be (added exclamation point):
<base href="[[!++site_url]]" />
