Found the reason and a solution too: The polygon direction (normals) in Cinema shows nothing abnormal. But I then opened  different .obj files of the same object with a text comparing program. I recognised when I change the order of the points in a polygon (face)  like this: "f X Y Z" into "f Z X Y" the distortion was gone. For there where not too many of those polygons I managed to correct them by hand. Helpful was this link: http://paulbourke.net/dataformats/obj/

thanks again