Oh, okay, it's ESC ... finally found out by myself - but not the key I'd choose to set if I were a game developer.
Thanks to everybody for not being helpful within almost 2 months 😛🤓, including Siren of course. Great community ...
You can customize the keyboard config into the game's commands options. By looking at all of the commands, it becomes logical that the key for erasing is the same as the one for cancelling, which is designed as "X" as opposite to "OK".
