症状

When visiting some pages, Internet Explorer warns about delivering non-secured content over a secured connection.

A variation of the warning may look like this:

This webpage contains content that will not be delivered using a secure HTTPS

Confluence is running over SSL (HTTPS) and is served by Apache httpd server with mod_pagespeed.

原因

Mod_pagespeed does not support HTTPS entirely. See the improvement request in Google's code repository. 

ソリューション