Here is one such page: https://www.mu-43.com/threads/79927/page-2
Unless I'm mistaken, the only non-secure content on that page is one of the attachments which was created before the switch to https. The URL for that attachment is http://www.mu-43.com/media/19766/full
... which gets automatically rewritten to https://www.mu-43.com/media/19766/full
... but still trips the browser warning.
My sites are photography forums, and I noticed that the vast majority of pages give this warning. Is there an easy solution for this?