"Failed to load mission"

[Doomdealer]

Hi, I'm trying to play ETO over a lan and get "Failed to load misson" errors from most of the missions supplied along with the installer. I have tried Era's 2 and 4 and only managed to get 2 missions to actually run.
Is this a common problem and can anyone give any advice on how to fix it?

All help appreciated as just getting into this great game!