I have a similar issue with the issue mentioned in this post. IE pops up a warning about mixed content of secured and non secured when using https with a page containing a progressbar control. Removing the progressbar removed the warning. That's how I figured the progressbar is causing the issue. Plus the fact that the progressbar creates an iframe.

This Stackoverflow question mentions that an empty iFrame causes the warning in IE. I don't experience this issue with Chrome, Firefox and Opera, the other non IE I tested with.

As a test I tried saving the page's html complete source, manipulate the iFrame and serve it using https from my machine but I got several javascript errors so I abandoned this method.

My app uses IE7 compatibility mode (for some other issue). My workaround was for this page which has the warning issue, I had it render in standards mode. The warning disappeared.

I am not sure if this is an IE issue, EO or both. My guess is that EO is rendering the iFrame in a manner which IE doesn't like... in a certain IE mode.

I am posting this for the user as a helpful hint where switching IE's mode can solve the issue and for EO developers to maybe look further into iFrame rendering.