I consistently received the same message yesterday when attempting to book on BA from a number of different East Coast USA departure points to AMS. It would calculate the points and taxes after I had selected available flights but then display the above error message instead of the payment screen. I tried different routings and then browsers to no avail so rang the MPC (sounded like MNL) and the itinerary I wanted was booked in about 10 minutes (after a hold of 4 minutes) and ticketed about 3 hours later.