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no error, it just imports the object complety in grey (the default color of imported meshes) with the  texture parameter empty

Then you can use Assetforge to add textures, export the results, then use them in your game

does assetforge allow to export the texture in png? 

I think so.  If not, export as obj, convert the texture to png and edit the mtl file to change the image file extension it's looking for.