For troubleshooting, I recommend
For trouble shooting recommend https://community.kde.org/Plasma/Wayland/Nvidia, checking whether all kwin backends are installed (kwin-wayland-backend-* on Ubuntu). Sometimes there's also a problem with the missing OpenGL backends of Qt. An easy check to see whether there is a problem with the proprietary driver is to try out whether the problem also exists using nouveau.
@Fleppensteijn @leopold @stevecrox
For troubleshooting, I recommend
For trouble shooting recommend https://community.kde.org/Plasma/Wayland/Nvidia, checking whether all kwin backends are installed (kwin-wayland-backend-* on Ubuntu). Sometimes there's also a problem with the missing OpenGL backends of Qt. An easy check to see whether there is a problem with the proprietary driver is to try out whether the problem also exists using nouveau.