Hi Hermann,
Better than a long story, see the screenshot. There's a kind of ghost window that pops up with the plugin GUI.
This happened so far with the JCM800 and GxAmplifier-X in latest Qtractor git (Version: 0.8.0.69git.e8b1d5).
Window manager is Flubox. That doesn't seem to happen in Ardour, so possibly a Qtractor issue?
Cheers,
LX