Graphic user interface draw errors when opening a plug-in in Wavelab 7 on Mac computers may be resolved by dragging the Wavelab toolbar from the top of the plug-in window (default) to the bottom of the window.
Ok, solved the problem with my external library finally. And the one with the gui in Wavelab was really easy. Did the workaround and shifted the wavelab toolbar from top to bottom. Now everything works just fine fine fine!