I also got
Hinweis: Fehlendes Signed-By im Eintrag sources.list(5) für »http://deb.debian.org/debian«
I had to complete in /etc/apt/sources.list.d/debian-backports.sources
the line
Signed-By:
into
Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg