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I've had the same trouble, it's also occurred when a country (human or AI) has produced a transport unit. Sorry to say I haven't found a permanent solution, only a temporary one: to reload a previous save. We were using Windows 7 as well. But I think it's an AoD thing. IMO a lot more attention is paid by AoD developers and programmers to single player, but then again I imagine many bug reports on Multiplayer have been resolved, we just don't see them in the update changes list (clearly).