Hmm, actually I'm not sure about that domain setting either. The phpBB 2.0
documentation isn't very descriptive, too
In addition the cookies got their own configuration with a "Cookie domain" setting.
I'm not sure whether cookies are required for being able to log into phpBB 2.0 forums - I think they are.
All cookies I have from sfml-dev.org (in Firefox) do contain only sfml-dev.org URLs, none from sfml.sourceforge.net, so they should be fine at the moment.
In case anybody here in this forum has some "timeout" from sfml.sourceforge.net here again that one should reload that page and then look into the page source of the HTML website and search for sfml.sourceforge.net. Maybe there are some leftover URLs in it which cause the "timeout".