LinuxQuestions.org - Repository with no release file and 404-errors

Hello everybody,
Just lately I seem to have an issue updating/upgrading my debian-stretch
system with apt update && apt full-upgrade. I wonder how to fix this.

Here comes the output on my KDENeon (Plasma 5.8.8) konsole:

Quote:

Ign:1 http://ftp.at.debian.org/debian stretch InRelease
OK:2 http://ftp.at.debian.org/debian stretch-updates InRelease
OK:3 http://security.debian.org/debian-security stretch/updates InRelease
Ign:4 http://deb.debian.org/debian stretch InRelease
OK:5 http://ftp.at.debian.org/debian stretch Release
Ign:6 http://ppa.launchpad.net/linuxenko/chkservice/ubuntu artful InRelease
Ign:7 http://archive.canonical.com/ubuntu stretch InRelease
OK:8 http://deb.debian.org/debian stretch Release
Fehl:9 http://archive.canonical.com/ubuntu stretch Release
404 Not Found [IP: 91.189.92.191 80]
Fehl:10 http://ppa.launchpad.net/linuxenko/chkservice/ubuntu artful Release
404 Not Found
Ign:11 http://repo.vivaldi.com/snapshot/deb stable InRelease
OK:13 http://download.draios.com/stable/deb stable-amd64/ InRelease
Ign:12 http://cdn-fastly.deb.debian.org/debian stretch InRelease
OK:14 http://cdn-fastly.deb.debian.org/debian stretch Release
Ign:18 http://repo.vivaldi.com/stable/deb stable InRelease
OK:19 http://repo.vivaldi.com/snapshot/deb stable Release
OK:20 http://repo.vivaldi.com/stable/deb stable Release
Paketlisten werden gelesen... Fertig
E: The repository 'http://archive.canonical.com/ubuntu stretch Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://ppa.launchpad.net/linuxenko/chkservice/ubuntu artful Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.