Ok, so despite having a dedicated server I hadn't bothered to switch bpsite forums from my old shared hosting.
Well I did that this week, and it turns out that Invision is too crap to handle PHP5.
So, after poking around trying to get it working, I figured it was simpler just to upgrade to a newer version.
Except Invision is no longer free, so I simply grabbed SMF and a converter, and after a bit of jiggerypokery with the database and conversion script, I got it working.
Welcome to BPSITE SMF Forums.