Trying to book MSY-NCE RT 5/24/2019-6/8/2019 (within the window of availabilty). The web site lets me select flights, and prices them. But when I get to the payment screen, it says "One of your flights is no longer available". I've tried several different flights and dates in both directions; the result is the same. These are code-share flights with BA. Is there some trick I'm missing?