I too was having a problem with that error and I called and the lady told me it was because someone else was buying tickets for the same price at the same time and there was a price change.
Got the same excuse as well the second time I called in to book over the phone.
Naturally, the first itinerary I booked over the phone didn't give me my online booking point bonus, despite the CSA's assurances that she would note it on the reservation so that I'd receive it. A followup email to B6 got that resolved (well, supposedly resolved, as the bonus points haven't posted yet), and I subsequently replied with detailed information about the problem and the workaround (and a friendly reminder to make sure I receive the online bonus for the second itin I had to book over the phone).
That was about two days ago. We'll see what response, if any, I get.