Does that show up during a screen test (hold the D key through powerup to run)?

Thanks for the tip. I'll try it and let you know.
Video recording of the error.

https://youtu.be/FKTuaGAKzxA

Restart 9:40

@ejn63​  Please judge for yourself. Connected a second monitor via HDMI, cloned desktop.

https://youtu.be/RTdcbioPW8Y