FBX export material issue

[guenther Malek]

After having baked GI into textures, i try to export my model as an FBX file using the (Standard)materials that got generated while Texture Baking.

It says: "Some materials in this scene may not be supported by certain applications and will import as standard gray materials instead."

When I apply a different material and then RE-apply the original material, it exports fine.

Does anyone know what that is? Thanks!