I find that if I leave Cubase up for a while (over night as an example) then the program will not close if I click the red x at the upper right of the Cubase menu bar. The project closes, but the Cubase menu bar remains, and is non responsive.
I actually thought that was a new feature: the X top right will close the project but to close the program you have to use the menu. The menu is still responsive in my case.
C10 at some point few months ago stopped closing. Updated to 10.5 coulpe of days ago and problem remains, thought it occurs randomly now and is not persistant.