Whenever I'm opening an existing scene which uses a custom Desktop, "Load Color Correction LUT" window pops up. I have this problem since at least Houdini 20.0, perhaps even 19.x or earlier. This issue seems to be harmless, but is very annoying, so I decided to address it somehow.

Why does this happen and how to prevent this window from popping up?

I think it happens whenever a desktop I switch to has Composite View pane displayed in the foreground.

Me too, very depressing.
It seems to happen when I open a project that has a problem with COP.