Error on simple Kitchenscene

[Alain]

I imported a Scene from Sketchup into Blender.
Did setup some Material (some inked to external Materialfiles *.igm).

When I hit F12 everything renders fine in Indigo RT.
Then I close Indigo RT and go back to Blender to change one Material which was linked to a *.igm File before to a Specularmaterial.
Then I hit F12 again and I have the following Error:
Error: [Indigo 2012-May-10 21:35:15] Writing to ID classes in this context is not allowed: Kue_Chrom_poliert, Material datablock, error setting indigo_material_external.material_name

And no, "Kue_Chrom_poliert" is not the Material I changed.

Does anybody have an Idea what I do wrong ?

Kind regards
Alain