@EvilSOB
unfortunately that doesn't fix the problem for me, but thank you for your suggestion.

@Hummel
That's the thing I don't understand here. I do some heavy vertex manipulations, I added the wireframe material to see if the mesh is realy modified. If I don't call fixnormals, the mesh is modified like it is supposed to be. But even if I use fixnormals a lot of frames later (or in the same frame), the mesh is reset to it's original state. And to be sure, no mdl has any frames (one of course).