Hello,
after applying "Reset system to factory conditions " in Settings>>System I found to my surprise /etc/apt/sources.list damaged in this way:
deb http://192.168.0.69:3142/ftp.de.debian.org/debian buster main non-free
deb http://192.168.0.69:3142/ftp.debian.org/debian buster-backports main
deb http://192.168.0.69:3142/security.debian.org/ buster/updates main contrib non-free
After removing the ip numbers and ports in every line manually “apt-get update” worked again without error.