On a Hetzner server with Debian 11 and HestiaCP 1.8.11 ready to upgrade to Debian 12:
# apt update
Hit:1 http://mirror.hetzner.de/debian/packages bookworm InRelease
Hit:2 http://deb.debian.org/debian bookworm InRelease
Hit:3 http://deb.debian.org/debian-security bookworm-security InRelease
Hit:4 http://deb.debian.org/debian bookworm-updates InRelease
Get:5 http://mirror.hetzner.de/debian/packages bookworm-updates InRelease [52.1 kB]
Get:6 http://mirror.hetzner.de/debian/packages bookworm-backports InRelease [56.5 kB]
Hit:7 http://mirror.hetzner.de/debian/security bookworm-security InRelease
Hit:8 https://nginx.org/packages/mainline/debian bookworm InRelease
Ign:9 https://mirror.mva-n.net/mariadb/repo/10.6/debian bookworm InRelease
Hit:10 https://packages.sury.org/apache2 bookworm InRelease
Hit:11 https://packages.sury.org/php bookworm InRelease
Err:12 https://mirror.mva-n.net/mariadb/repo/10.6/debian bookworm Release
404 Not Found [IP: 195.65.111.42 443]
Hit:13 https://apt.hestiacp.com bookworm InRelease
Get:14 http://mirror.hetzner.de/debian/packages bookworm-backports/main amd64 Packages [172 kB]
Reading package lists... Done
E: The repository 'https://mirror.mva-n.net/mariadb/repo/10.6/debian bookworm 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.
# cat /etc/apt/sources.list
## Note, this file is written by cloud-init on first boot of an instance
## modifications made here will not survive a re-bundle.
## if you wish to make changes you can:
## a.) add 'apt_preserve_sources_list: true' to /etc/cloud/cloud.cfg
## or do the same in user-data
## b.) add sources in /etc/apt/sources.list.d
## c.) make changes to template file /etc/cloud/templates/sources.list.debian.tmpl
###
# See http://www.debian.org/releases/stable/i386/release-notes/ch-upgrading.html
# for how to upgrade to newer versions of the distribution.
deb http://deb.debian.org/debian bookworm main
deb-src http://deb.debian.org/debian bookworm main
## Major bug fix updates produced after the final release of the
## distribution.
deb http://deb.debian.org/debian-security bookworm-security main contrib
deb-src http://deb.debian.org/debian-security bookworm-security main contrib
deb http://deb.debian.org/debian bookworm-updates main
deb-src http://deb.debian.org/debian bookworm-updates main
## Uncomment the following two lines to add software from the 'backports'
## repository.
##
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
# deb http://deb.debian.org/debian bookworm-backports main
# deb-src http://deb.debian.org/debian bookworm-backports main
# cat /etc/apt/sources.list.d/mariadb.list
deb [arch=amd64 signed-by=/usr/share/keyrings/mariadb-keyring.gpg] https://mirror.mva-n.net/mariadb/repo/10.6/debian bookworm main
Is the URL correct?