Since the forum redirects by default to HTTPS the theme "GDNet 3.4.1" have issues. It looks like some content is still served on HTTP what results in security warning about not all content loaded, malfunction of javascript and slightly altered layout. Reloading the page with "allow all content" button solves the issue until I navigate to any other page (another subforum, specific post, ...)

I don't experience this issue using the "GDNet 6.0" theme. Testend on Win10, IE11.