I'm also having an issue with this, trying to book a LON-EDI trip. I can see (on Seatspy and BA's own 'reward flight finder' tool) availability for the dates I want, but when I click on the 'get flights' button from the standard search, or choose the dates on the availability calendar and press continue, I get the "sorry there was a problem with your request" message. It's the same on a PC or iPhone and I've tried this several times over the last few days. Other dates seem to work OK so I don't really understand what's causing the problem. Any clues?