1. It could be a theme issue. Verify that in your theme's header.php file, you see the line:

wp_head();

You should always have wp_head() just before the closing tag of your theme, or you will break many plugins, which generally use this hook to add elements to such as styles, scripts, and meta tags.

If the line is not there, just add it before the closing tag of your theme.

2. It could be because the web server hosting your WordPress domain is unable to resolve your WHMCS domain and basically can't pull the screens from WHMCS. This is something you should verify with your hosting company.