WebEngine PageWindow is blank

[Topographics-dev]

Hello,

I am installing QGIS 3.40.12 on Windows 11.
After that, I activate PyQtWebEngine with OSGeo4W Shell ("pip install PyQtWebEngine").
Now the menu "Qgis2threejs Exporter (WebEngine)" exists, but when I click on it, only an empty window appears.

The context menus work, but no menus, etc., are displayed.
It seems that no web rendering is being done with PyQtWebEngine. (The Plugin works with WebKit!)

Any help is appreciated, thank you!