The skin must not be indexed, but normal 24 bit RGB. However there is no real .obj standard - there are many variants. If in doubt, export the file in a standard format such as FBX, and convert it to .mdl.

Yes I finally got it working with Blender , thanks so much jcl and 3run for helping to get me on the right track.
In short; the problem was that the model got exported with a material attached to it.
I will make a short guide for this cause I think this could be handy for others too. I will post a link to it here soon.

Now I still need to figure out how to disable exporting materials with Google Sketchup. And probably with some other often used programs. found the solution (will put it in the guide)

guide link