Hi Marc,
Open the obj file in a text editor and search for "mtllib". This line refers to the mtl file. I suspect that maybe you have a full path to the mtl file there, something like:
mtllib C:\Users\Marc Grebhadt\Desktop\CL01_Abies_concolor_White_Fir\CL01y.mtl
Assuming that the mtl file is in the same folder as the obj folder, as you have, you may have more luck if you change the line to look like:
mtllib CL01y.mtl
I think the problem is the space in your user name. The obj format doesn't officially support spaces in the mtllib filepath, but plenty of applications do that and it's a bit of a problem for loading applications like TG2 which follow the spec closely. We need to loosen this up. I don't know if Xfrog allows this, but if you export the obj file so it uses relative paths for the mtl file and such then it should be less of a problem.
Regards,
Jo