Hi uli - is it possible that you have a file composed only of wireframe curve objects like only lines and circles and not any surfaces or solids?

If so then that would result in the behavior you are describing - both STL and OBJ formats are polygon mesh formats and in order to generate polygon mesh data you have to have surface or solid objects, not just wireframe curve objects.

If you have drawn some curves, you need to construct some solids from them first before you'll be able to generate an STL file.

If that's not it could you please save your model file that you are having problems with to 3DM format and post it here or e-mail it to me at moi@moi3d.com so I can get a better idea of what you might be running into?

- Michael