Hello. After logging in at the main site and switching to the forum the "log in" button is showing and I am not logged in. But after clicking the button the forum refreshes and my session from the main site is accepted an I am loged into the forum.
Does it do it in incognito mode? There was an update on the forum as well as a recent incident and as such cookes are invalidated, that may be causing your login issue.
I was having this trouble yesterday, although every time I clicked on 'log on' it would refresh the page (both main and forum) and fail to present me with the actual log on page. After removing the cookies the problem was gone =]