I think it depends when you try to do it. It is much more likely to fail at 3pm on a Sunday than at 1am at any time. My understanding of the reason for the failure is as follows (not guaranteeing this is correct).
AB works by starting from the most recent postings and working backwards. So if you try to display page 2 it has to find 40 entries while page 3 requires 60 entries etc, etc. Hence the further you try to go backwards the more entries it has to find and the longer it takes. And the more people that are using AB (eg Sunday afternoons) also makes everything slower because presumably there is only a fixed server capacity.
But the programmers set a time limit on any such �searches� so as to avoid using up too much server time. Hence the further you try to go backwards or the busier AB is then the more likely it is that you will fail due to a timeout. I believe the only solution (without re-writing AB) is greater server capacity.