I got the same problem. Played just fine, logged out and came back in. Now the server indicates a discrepancy between versions, does that have anything to do with it? I used not the latest beta of ARMA 2 OA, (was told to do so because the server used an older version) but now I upgraded to the latest. Either way it won't work, and I still get an error message about version compatibility. I spend as much time fiddling with DirectX DLL's patch versions etc as I do playing the thing...