I had a similar problem in DNN with mixed mode content error.
The problem came from a custom URL rewrite module we have that was not returning https correctly for an iFrame on the page. So the page was https, but when DNN was rewriting the URL for the iFrame with our custom url rewrite module, there was a logic error in our code and the iFrame was coming as http. This also happened when we upgraded DNN to v6, so not sure if something in the core had changed that threw our logic off?
Anyway, check your URL rewrite module is behaving when rendering the URLs in the menu.
Also check for any iFrames or code script blocks (jquery, jqueryui, etc) that are not getting their content from a secure (https) source. Maybe even images?