Uwe - if you could post a screenshot of your plugin Settings window - and post exact (ie. keystrokes/menu commands) for producing this error then I will have the best chance of replicating it and fixing it.I have a default scene that is empty except for the ground plane and a (invisible) cube from the Poser primitives. That cube is for scaling reference and it's visibility is switched off because I only need it temporarily. It seems to me, that this causes the problem. If I load up Poser with my default scene and turn the visibility of the cube on, then everything works fine in the plugin. The error occurs when the cube is invisible.
Hope, that helps, if you need more info, just ask.
Paul