On user explained:
Windows XP visitors using IE 8 on several of my Adsense sites see an error message "Internet Explorer has blocked this website from displaying content with security certificate errors."
Without AdSense loaded there is no error. Turns out you have to update the root certificates for your computer at windows update manually if you have an XP computer. Windows doesn't automatically update these certificates for XP users.
This is supposedly only happening on a specific version of Internet Explorer and only happening one web sites that contain Google AdSense.
It might be related to a Google change or maybe a Microsoft change. It is unclear at this point.