I used to use Kubuntu 20.04 and did a complete fresh install of Kubuntu 21.04.
All is fine so far except that the windows key refuses launch the Application menu.
I ave no clue where to set this shortcut. In Settings I found no corresponding entry.

Digging around a bit I found out that I can assign a shortcut with:
Right click to start button -> configure Application Launcher" -> Shortcut
Here I can assign the Meta(Windows)-Key, but only in combination with another key.

After that, however, the windows key alone does it's work.
Looks like a bug to me.

I am using the current fresh installed Kubuntu 21.04.