You cannot use Ctrl+Alt+Space as a shortcut key because it is a zoom out shortcut key.
Better to use another option for that.
1.) So why can I choose it CC?
2.) Why does it work in CS5 (while its a zoom shortcut too)?
Yours is a quick response, but doesnt explain the logic behind this difference. I should work too. Or be documented to work not. Or be not choosable. IMO.
I am using cs5 when i try to create a short cut key as Ctrl+Alt+Space for Third Space, nothing is coming out, it doesn't allow me to create a shortcut key as Ctrl+Alt+Space for Third Space but when i try different one it accepted. Hope you are on the wrong track. (Ctrl+Alt+Space is a key for zoom out by default it cannot be changed).
And one thing you cannot use it which is already created the short cut key.
I hope you are mistaken.