Greetings @fdortu,
The issue here is that you’re setting QT_QPA_PLATFORM="xcb" for your third-party GUI. This means Qt will use the XWayland x-server to display this GUI. By default x-server type applications don’t work for root user. This is why you’re seeing this kind of behavior.
There is a workaround for this however. For XWayland it doesn’t seem to disallow specifically the root user. What it does is check that if the user launching the GUI application is the same user that started Weston/Wayland. This means if we start Weston/Wayland as the root user then it should work.
I launched the Weston container adding this environment variable to it WAYLAND_USER=root. In another container I launched an example Qt app with QT_QPA_PLATFORM="xcb", and it worked! Where before it failed with a simlar error as your third-party GUI. Give this a try and let me know how it works out.
Best Regards,
Jeremias