The reason for d3dx9_30 was that this DLL is available on a majority of systems.

Of course I don't know if the bug or whatever it is was fixed in any DirectX version. As it seems, it only appears on very few systems that only have XP in common. The only thing I know is that it's not a Gamestudio bug and that I can not reproduce or test it. Therefore, using a different DX version is the only thing I can do at the moment. If someone who has this bug volunteers, I can send him a special engine with the latest DirectX version for testing.