I had the same issue as described in this thread. It was a code shared flight with Air Canada. After putting in the payment info, I received the error message. Tried different browsers. Same issue. I finally called. The agent also had the same issue. Was put on hold for a while. When she returned, she explained that their system had not been updated to reflect the fare on one leg was no longer available. Had to book a different flight for that day. You would think with today's technology available seats would be in real time.
