Original cause is not **RControl**.
Moving the advanced build mode to **J** for example, works properly.
Pressing **RControl** while having the //Advanced Build mode// enabled locks the mouse and removes the cursor.
Further investigation showed:
The **RControl** key is used in the //Sticky Cam Mode// (double tap right shift), to **unlock the mouse** again **while the chat is open**.
@Lancake found related code in //CameraMouseState// in function //isInMouseControl//, reason for this functionality may have been for the old chat system, to adjust your camera while chatting.