The plasma blur effect doesn't work after I start a new plasma session. If I then disable blur in desktop effects, click apply, then enable blur and click apply again, then the blur effect works correctly.
Please provide output of qdbus org.kde.KWin /KWin supportInformation
https://paste.kde.org/pmpsdxfmw
To use the Plasma blur effect you also need to enable the contrast effect.
The contrast effect was ticked halfway, just like the blur effect. I fully ticked the contrast effect and restarted plasma, but the blur effect is still not working on startup.