"non-optional space"
I am not familiar with this.
Do you mean non-breaking space?

There is already a keyboard shortcut for that: Ctrl+Shift+Space.

If you mean something else, Ctrl+Space is Clear Character formatting (revert to default paragraph font) by default. This is a very valuable command. You may want to use a differnt shortcut.

That said, where did you store the keyboard shortcut?
By default, shortcuts are stored in the normal template but they can be stored in any template or document.

Did the shortcut ever work for you?
If so, then, if saved to the normal template, it is likely that something is preventing changes being saved to your normal template.