My brother has that problem, too. He needs to answer the security question every single time he logs in, even when he does it from the same PC. According to phone support, some accounts are bugged that way and need to answer the question every time and "they are working on it". He was told that back in June, though. Hopefully that won't mean the people that suffer from that bug will get spammed by these new security question e-mails every single time he logs into the game or forums.
Try changing password. It worked for me.