I had a similar problem with Tapatalk and AoS a few months ago, I think it is to do with the cache for it becoming corrupted on the device, this is were the user id and password will be stored so that when you re-open the Tapatalk app it looks for the Forums logons that you are subscribed to in Tapatalk and then tries to log you in automatically. Obviously if the cache with the userid and password has become corupted then you will not get in. By entering the Username and not email address you probably have refreshed the cache for the AoS logon in Tapatalk and presto in you get.
I hope that makes sense, all you needed was an IT Genius.
By the way are you still selling the odd vinyl I know we did speak about Kula Shaker some time ago.