Thanks Electron,
Especially for trying, much appreciated!
I finally figured out what seems to be causing the problem.
When messing with the materials in Blender, I assigned all faces to a single material and things started to work.
When I changed materials, at some point it started to go wrong.
Sometimes I got material not found 'Undefined' (while everything was assigned, I made sure of that).
Sometimes I got the emtpy message, and no shape was generated.
I found out that either error happened when I started mixing nadeo material names with mapped ones!
So when you have materials:
Blender material names:
StadiumPlatform
StadiumPlatformFloor
mesh.xml:
(no <material/> with above names)
All is fine, it uses Nadeo materials.
When you have:
Blender material names:
My_StadiumPlatform
My_StadiumPlatformFloor
mesh.xml:
<Material Name="My_StadiumPlatform" Link="StadiumPlatform"/>
<Material Name="My_StadiumPlatformFloor" Link="StadiumPlatformFloor"/>
All is fine, it uses Nadeo materials.
But! When you have:
Blender material names:
StadiumPlatform
My_StadiumPlatformFloor
mesh.xml:
<Material Name="My_StadiumPlatformFloor" Link="StadiumPlatformFloor"/>
Things start to go south!
The solution is easy, just use either/or.
Now my next issue is:
I now changed (following line works fine):
<Material Name="My_StadiumPlatform" Link="StadiumPlatform"/>
Into:
<Material Name="My_StadiumPlatform" Model="TDSN" BaseTexture="StadiumPlatform" PhysicsId="Metal"/>
But now I'm getting:
NadeoImporter 2017-05-18_12_54
File not found: 'Items\Paljas\blender\StadiumPlatform_D.dds' File not found: 'Items\Paljas\blender\StadiumPlatform_D.tga' Created :user:\Items\Paljas\blender\halfloop_00.Mesh.gbx Created :user:\Items\Paljas\blender\halfloop_00.Shape.gbx
Back to debugging

. Thanks everyone for the help, the initial issue is solved!!
EDIT: Above problem also solved:
Do not combine link/model in the same xml. Use always either one or the other:
Wrong:
<Material Name="My_Material1" Link="StadiumPlatform"/>
<Material Name="My_Material2" Model="TDSN" BaseTexture="StadiumPlatform" PhysicsId="Metal"/>
Correct:
<Material Name="My_Material1" Model="TDSN" BaseTexture="StadiumPlatform" .../>
<Material Name="My_Material2" Model="TDSN" BaseTexture="StadiumPlatform" PhysicsId="Metal"/>
The red message still appears, but the materials now work OK because of the hack option of ConvertAssistant.
Note also that using the <Material ... Model=.../> approach you now need to use ConvertAssistant, because you need it's hack feature for this to work, even if you don't intend to change e.g. the PhysicsId. You cannot use the bare NadeoImporter only anymore.
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