Close button is not working

[filo]

Hi there, welcome to the forum!
Does it happen also when windows are not maximized?
If not, try this:

The top panel of the application window disappears when deployed to full screen

Try opening ~/.config/kwinrc and go to section [Windows]. Set BorderlessMaximizedWindows=false, logout and login.

Otherwise try reapplying the Dr460nized Global Theme.