Thank you for your general suggestions. The problem is, I'm running on a mac, so Poseray is not available for me (AFAIK).
To say it again: The project (tgd) works fine (only the warning message "disturbs") and the render finished without any problem.
How sensitive is the Pixel Plow render software? Will it abort the render, when this warning message appears or will it continue?
In fact it's "only" a message, not an error that will crash the render job...
Well at last it's probably "no risk no fun".