standalone Opus viewer window -> yes it is definitely the Opus viewer
only from inside Opus does it open in the viewer
But I now realized: It happens because of this plugin: Universal Graphics Viewer Plugin (Universal Graphics Viewer Plugin). If I deactivate it, all is back to normal. Is there a way of how to use the best of both worlds? Or even is there a better xcf/gimp-thumbnail creator (this plugin uses only the top graphics layer from inside the xcf format - horrible)?