i have had the same problems sometimes. Although i m not quite sure whats the real cause for it...but i consider that it has nothing to do with password or username.
We might have problems log in when the forum is under some mantainance or the network is too busy. It generally happens at around 6 or 7 pm GMT.
so we need to click "refresh" hard (lol) ,or just try to avoid that period of time in a day.
Hope it helps