Whenever I try to attach my authenticator, from the computer or phone, it says the code was invalid. I cleared the data from the Battle.net app multiple times. Clearing the data from the app gives you an entirely new authenticator serial and restore codes. Every time the code fails, I have to disable the authenticator through the Blizzard support page. I submitted a ticket two days ago, then directed me to this forums page.
This started to happen around a year ago, but recently just started trying to deal with the problem.
Edit: Problem is fixed. Clear the data from the Battle.net app and RESTART your phone. Log in through the app and you should be good.