This is happening to me. It's very annoying.

I'm trying to book a fairly complicated J class itinerary for 2 people. Takes ages to get to the final page and then gives the 'Sorry, the system was unable to complete your booking. Please try again.' message.

I'm using Amex, and I've confirmed with them that they haven't received the payment request.

I'll try again tomorrow, and then try phoning.

Cheers,
Rick