In case you use the helfertoolctl Debian repository, you might notice the following error:
The following signatures were invalid: EXPKEYSIG FA1023F9F6AC494F Sven Hertle <hertle@fs.tum.de>
The key was already renewed some time ago, but you need to update it on your system:
sudo apt-key adv --recv-keys FA1023F9F6AC494F