Hello,
The terrain editor registers in the OS that it wants to listen for that hotkey. Unfortunately the exact mechanism used is long forgotten.
I understand that RGC also registers these hotkeys and therefore interferes with terrain editor.
Maybe there is a way to (1) disable RGC's ability to receive keyboard events or (2) send a hotkey event directly to terrain editor. Unfortunately I am not familiar with the way how hotkey events work.
Have you reported this problem to the RGC?