(I will answer in English to try to help you, but you can wait for answers in German.)
These error messages are from your OS, not from Pi-hole.
You are running a very old OS (Stretch - also known as Debian 9).
This OS was released in 2017 and it can't be updated anymore because it reached its End Of Life a long time ago.
You won't be able to update this system.
Also, the current Pi-hole versions (v6.x) can't be executed on Stretch.
Suggestion:
create a Pi-hole backup (teleporter) using the Web Interface or the command line (pihole -a -t);
save the backup on a different machine;
install a newer OS. The current version is: Raspberry Pi OS 13 (Trixie);
install Pi-hole on the new OS (curl -sSL https://install.pi-hole.net | bash);