We're doing our best to get this resolved as quickly as possible. It seems the problem is affecting a broader range of users than before, so maybe there's some clues in the most recent build. Chatting to Unity and also tweaking code on our side where possible -- if you keep an eye on the forums you'll probably hear about further developments in this area.
Thanks for the patience, hoping this current batch of problems is just a bad Beta crisis memory soon!
Funnily enough (it's not funny at all, I'm just putting on a brave face so that I don't go jump off a bridge) the game will only actually BUILD on 1 machine out of the 6 here at the office. This is part of the problem that we've been talking to Unity about, so far they're not having the same problem anywhere... This points to some sort of secondary side-effect-based information messing with the game. Tracking those down is always incredibly hard.
I am not looking forward to the next couple of weeks.