First of all: sorry if this is in the wrong category, but there's none for forum.xfce.org. Typically when I enter forum.xfce.org in the address bar of Firefox, https://forum.xfce.org is being loaded. This is also true for the most time when http://forum.xfce.org is entered, a redirect to https://... follows. However every once in a while I'm landing on http://forum.xfce.org, which shouldn't be the case. Are there differently configured servers behind forum.xfce.org, which causes this odd behaviour?
Good catch, there was no redirect from http to https on forum.x.o. But as https version send Strict-Transport-Security header, browser keep it and always reuse https version. However, on first try in http (new browser or never visited the site), the redirect is not done. Fixed by a redirect 301 http->https