Hi @hellpunch,
You are using a CSS stylesheet from a HTTP domain on a HTTPS site. Ie, your site is https://www.yourdomain.com/ and the stylesheet is loaded from this URL:
https://www.yourdomain.com/wp-content/uploads/us-assets/www.yourdomain.com.css?ver=d81a38c0
It works when PhastPress is enabled, because it retrieves the stylesheet and inlines it into the page. But when you disable PhastPress, the browser has to retrieve the stylesheet itself, and can’t because it is not allowed to use HTTP resources on HTTPS pages.
You have to fix your configuration so that all resources (CSS and JS) are loaded from HTTPS. This might be your “site URL” setting or a setting in your theme. You can try adding a line like this to your wp-config.php:
define("WP_SITEURL", 'https://www.yourdomain.com');
–Albert