It seems that you cannot be logged into stackoverflow and stackoverflow careers simultaneously. If you are already logged on to stackoverflow and try to log on to careers, it will bring you back to the careers homepage and not give you any kind of indication that the login has failed. Is this intended? Is there some reason that you can't be logged into both at the same time?
Do you have the same sign-in on both sites? Due to the new central authentication feature, if you have different sign-ins you may experience weirdness, and in fact may be unable to sign in to both at the same time.