When embedding links into forum posts, the post shows up with the link converted to https, REGARDLESS if you used plain http for a REASON. This is not wise since not all sites are HTTPS capable and this leads to bad links and user confusion. It should use the scheme provided by the user and not blindly change it.
link that gets converted to https by your forum and fails to load because the site doesn't have that link available by https
Look at the url bar in the browser, change https to http and the page loads.