Custom Keyboard Shortcuts Ignored by Engine

Engine completely disregards custom shortcuts, even when they do not conflict with existing shortcuts.

I want to change timeline Playback to a logically consistent pair of keys - Home/End, Up/Down, PageUp/PageDown, CTRL-leftArrow/CTRL-rightArrow… ANYthing but the default combination of Up and Ctrl-UP, which makes no sense to me.

I have entered a dozen variations of combinations and they are all simply ignored. Any help would be appreciated.