If you access the Magnolia Backend with HTTPS using an IE Browser: u get this warning:

"This page containsd secure and non-secure items..."
http://farm3.static.flickr.com/2263/2330340408_63a47989da.jpg?v=0

If you look here: http://weblogs.asp.net/rchartier/archive/2008/03/12/ie7-this-page-contains-both-secure-and-nonsecure-items.aspx
somebody seems to have found the cause.

The Magnolia Backend seems to use an iframe without "src" attribute - the "src" attribute is then set by JS. But on page-load the IE interpretes this as unsecure.