I have mirrored instances of objects in my scene, instead of mirrored copies, and I have a problem to paint on the instances with the 3D Paint Tool.

I can easily assign different materials with the according different file textures to the mirrored instances. But as soon as I try to use Mayas 3D paint module, I'm getting this warning here (and subsequently I can't paint on the mirrored instance):

// Warning: Some surfaces have no shader group, assign them and try again.

I tried my best to find a reason and a fix for that in the Hypergraph, including displaying all input and output connections of the mirrored instance in question. A shader group seems to be there (http://www.landgasthof-waldschaenke.de/sculptytutorial/3DPaint_and_Instances_Problem_Graphs.jpg).

Does anybody know how to work with Mayas 3D Paint module and instances of objects?

Thanks in advance for any insights on this, cheers,
-N