This is the problem I had trying to buy my son's ticket through his fan number/log in (it wasn't showing on mine). I found that if I did this on his so that it was showing in the basket, then logged out and back in to my fan number log in, the ticket was still in the basket and the red message...
Managed to buy my ticket at lunchtime without any problems, but my son's wasn't listed on my fan number login. Figured I had to log in with his fan number, sure enough there it was. When I tried to check out I got a message about having selected too many tickets for this fan number and couldn't...