Okay, I got it
When Wings creates an image plane, the plane is actually a flattened 6-faces polygon whose sides are so small that you dont even notice them. Those faces are not UV mapped, but S3d requires all the faces to have UV coordinates.
The quickest "fix" for your map polygon is as follows:
- select it in body selection mode, switch to face selection mode, right click, Tesselate => Untriangulate.
- Press space bar to unselect all and in face selection mode select one of the big faces (the one pointing down, whose texture is mrrored)
- Press the "F" key on your keyboard. Doing this will extend the face selection to all the faces sharing an edge with the face originally selected (i.e. to the flattened, non-UV-mapped faces encircling the map polygon). If all worked correctly, at this point you should have 5 faces selected (check it from the left corner of the top info bar).
- Unselect the lower face if you want to keep it, right click and select "Hole" to remove all the faces still selected. This should leave you with a geometry with only 1 or 2 faces.
- Select all the remaining faces, triangulate them again and export to obj file
