Arma 2: Error compiling pixel shader

You're probably running the Official ArmA 2 Beta modfolder named "beta", while you did not adjust your shortcut appropriately e.g: "C:\Program Files\Bohemia Interactive\ArmA 2\arma2.exe" -mod=beta

should be: "C:\Program Files\Bohemia Interactive\ArmA 2\beta\arma2.exe" -mod=beta