Hi TIG,
thanks for your support, that's not self-evident!
You have been right, it was a 'Compatibility Files' issue as you recommended. Your plugin 20111113 is not working if sketchyphysics 3.2 is also installed. The older version of your plugin in combination with an older sketchyphysics version worked. Now I deleted sketchyphysics and your plugin is working fine!
Greetings