Keyboard shortcut (App Shortcut) taken? Cannot use "^Z" When I try to add a Shortcut like this

System Preferences->Keyboard->Shortcuts->App Shortcuts->+->Some title->

I cannot use ^Z as Keyboard Shortcut.
It is automatically replaced by 


Shift (illustrated as arrow-up)
followed by
Command (illustrated by the Command key symbol)
followed by a
Y


I guess this is because the shortcut is taken. But I couldn't find where. I checked all the other item above "App Shortcuts", i.e. "Accessibility", "Spotlight", ...

How can I check which app is using it and how can I force the system to use my shortcut?
just a guess… but sounds like something might have a custom map to "Undo" because Cmd/z &amp; Cmd/shift/Y are alternates for Undo. 

For it to be auto-replacing inside system prefs would make me think it is not a system key command, but a third party, Karabiner, TypeIt4Me etc