How can I have the secret shared key ("code / key" or QR code) displayed again to secure it?
There is no option to display the QR code or Secret Shared Key again. If you still have access to your 2-factor authentication, you can deactivate it in your account settings under 'my Bitcoin.de' → 'Settings' → '2-factor authentication'. Subsequently you can reactivate 2-factor authentication. A new Secret Shared Key is generated and displayed, which you should note down.
Einen Moment bitte.