If the page is using https you will get this error if anywhere in the page is an image or javascript on the page with src='http://someurl'.
To fix it you must view the source of the page and find the offending items and figure out where they come from so you can fix them. You fix them by using either a relative url like src='/someurl', or by using src='https://someurl'
This applies to any element that has src attribute like iframes also.
Hope it helps,
Joe