Does Internet Explorer annoy you with this security warning?
This message appears when you’re using a secure connection (https://) and the website is trying to load non-secure (http://) elements, too. The culprit was easy enough to find (using HttpWatch or similar tools):
What made this case unusual is that it wasn’t a server-side problem. Instead, the DivX HiQ plugin + Web Player Beta installed on the client’s machine was causing the issue on every single secure page. The warnings disappeared after uninstalling DivX HiQ (disabling the plugin should work, too):
So, if Internet Explorer is suddenly giving you these warnings, check if you’ve recently installed DivX.