@Lanzi: Sorry, but this can`t work.[...] I have this entry:
Types: deb
URIs: https://ftp.gwdg.de/pub/linux/siduction/extra/
Suites: unstable
Components: main contrib non-free non-free-firmware
Signed-By: /usr/share/keyrings/siduction-archive-keyring.gpg
The following line is missing between “Components” and “Signed-By”:
Enabled: yes
And then, as hendrikL wrote, check whether all key files are stored in the
/usr/share/keyrings/ directory.
In addition:
The file that hendrikL posted is
deb deb-src. I assume you don't need deb-src.
Then remove deb-src, or if only deb-src is entered, set the value
Enabled to “no”.