This forum bug still exists from the very release of GW2. Sometimes* you are logged out of the forum and, no matter what link you follow, you are always redirected to the login page, even if all you want to do is read and don’t need to bother logging in at the moment. Only by deleting all cookies or logging in again you can view the forums again.
*I am not sure what qualifies here; it may be time since last posted, time not having visited the forum (like browser closed or computer turned off), or time since login.