I solved it by connecting the two meshes on export, but I don't remember this step necessary before.
Might be wrong of course.
!!!!!!!!
Edit
It did not solve my problem, because now with 2 texture sets instead of 1 DefaultMaterial, my previous work on the project does not show up. Exporting as obj gives me only 1 texture set.
How do others deal with this?
Reggards, Robert