Sometimes when entering a sub-forum, or a thread, the site redirects to the forum root. My impression is that it's after I haven't used the site for a while (minimally a few hours or a day), and usually when I arrive from a search engine or when using a URL directly. Pasting the same URL a second time shows the page correctly. The problem may also affect other site areas (*.forum.nokia.com).
It happens with multiple browser flavors, and it's not new, it's been like that for at least months. I'm not fully sure of the when and how because it doesn't happen often enough, but the above is my general impression.