I was having this exact problem last night (ie about 12 hours ago). I've just logged out and logged back in again, and the forums are back to normal now (this didn't work last night... i suspect there was some issue with passing credentials to the forum site so people who had stayed logged in were okay but anyone who'd logged out couldn't get back in again).
Anyway, if anyone's having issues still... try logging out and then logging in again...