HTTPS security is compromised by (null) - what causes this?

HTTPS
security is compromised by (null) - what causes this?

I am encountering the following error in IE 11:

HTTPS security is compromised by (null) - what causes this?

I am creating a popup (iframe) by clicking on a link, and this popup is causing the error.

I have read up on this error, and I understand that it is caused by mixed content. I have checked and all file includes also use https. Usually the specific file that is compromising security is mentioned, but in this case it is (null).

Any ideas?

Answering my own question after an interesting debugging journey:

It turns out that the popup that was not displaying due to the HTTPS security error wrote a piece of html in the body onload event handler. In this html snippet were two links with href=""; IE 11 sees
this as a security issue.

To fix I replaced href="" with href="javascript:;" (see this
post). http://stackoverflow.com/questions/20516612/https-security-is-compromised-by-null-what-causes-this