Transparency Effects in KDE Plasma, not supported by color scheme

I had same issue but I got it solved when I changed rendering backend to XRender for my Nvidia card.

System Settings ---> Display and Monitor ---> Compositor.

this can be solve by changing the option of compositor:

There were also several errors related to this same issue

I had the same issue recently with KDE Plasma. When I went to 'System Settings -> Display and Monitor -> Compositor'.. there it stated that opengl detection was off, due to opengl causing KWin to crash in the past.

I switch back from opengl 3.1 to opengl 2.0, and re-enabled it... it is working (at least for now).

I think it is similar to this issue: https://forum.manjaro.org/t/kwin-crash-after-changing-rendering-backend-to-open-gl-3-1/30827