One cause for a mixed content warning like this is if the page in question contains links that use "absolute URL's" that don't contain an "https://" prefix (note the "s"). An absolute URL is a web address that contains full prefix with either "http://" or "https://". A relative URL is one whose address is relative to the root of your NetCommunity site and doesn't include a prefix. Relative URL's are considered secure since they refer to content within your site.

If you create a page on your NetCommunity site that is fully secure when you browse to it directly and the page has no absolute URL's, you may find that if you use this page as your site's home page, when navigating to it via your site's main URL, you now see this mixed content warning.