I do think that the issue may be at least partly down to the website. When costing up a LHR - OZ -NZ trip I ntermittently get the error message quoted by the OP. It never comes up on a simple LHR-SYD rtn but will do so on occasions when multiple destintions are selected - even as simple as LHR-SYD-CHC-SYD-LHR.

I tend to go away and do something else, come back an hour or two later and ten the site plays nicely for a while