Dear Maintainer,

I switched mirrors in Synaptic package manager (Software & Sources) from the
main mirror to one of the faster ones, like Switzerland or Slovakia.

After the change I updated the lists using the Synaptic Update button.

Error message:

LANG=C sudo apt-get update

Err:16 http://ftp.sk.debian.org/debian bookworm-security Release
  404  Not Found [IP: 195.80.174.186 80]
...
Reading package lists... Done
E: The repository 'http://ftp.sk.debian.org/debian bookworm-security 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.

It clearly happened after the changeover to the Slovak mirror!

I checked, the contents of the sources.list in /etc/apt/ changed to

deb http://ftp.sk.debian.org/debian bookworm-security main contrib non-free
non-free-firmware
deb-src http://ftp.sk.debian.org/debian bookworm-security main contrib non-free
non-free-firmware

Whereas, it is the right contents, that it used to same as like earlyer

deb http://security.debian.org/debian-security bookworm-security main contrib
non-free non-free-firmware
deb-src http://security.debian.org/debian-security bookworm-security main
contrib non-free non-free-firmware

As an aside, but interestingly, the bookworm-backports mirror has not been
changed to Slovak, it has remained original

# bookworm-backports
# https://wiki.debian.org/Backports
deb http://deb.debian.org/debian bookworm-backports main contrib non-free non-
free-firmware
deb-src http://deb.debian.org/debian bookworm-backports main contrib non-free
non-free-firmware

From saved file (sources.list.ORIG), I restored the mirror to the original and
working URL,

# bookworm-security
deb http://security.debian.org/debian-security bookworm-security main contrib
non-free non-free-firmware
deb-src http://security.debian.org/debian-security bookworm-security main
contrib non-free non-free-firmware

majd frissítettem. And everything is fine

This is the result I was expecting, the improvement, of course. After all, I
found the error and corrected it using the official method.

However, I first suspected an fresh update (here the mirror is good),

sudo apt-get upgrade
Csomaglisták olvasása... Kész
Függőségi fa építése... Kész
Állapotinformációk olvasása... Kész
Frissítés kiszámítása... Kész
Az alábbi csomagok frissítve lesznek:
  gir1.2-javascriptcoregtk-4.0 gir1.2-webkit2-4.0 libjavascriptcoregtk-4.0-18
  libjavascriptcoregtk-4.1-0 libwebkit2gtk-4.0-37 libwebkit2gtk-4.1-0
6 frissített, 0 újonnan telepített, 0 eltávolítandó és 0 nem frissített.
Letöltendő adatmennyiség: 53,7 MB.
A művelet után 12,3 kB lemezterület kerül felhasználásra.
Folytatni akarja? [I/n]
Letöltés:1 http://security.debian.org/debian-security bookworm-security/main
amd64 gir1.2-webkit2-4.0 amd64 2.40.5-1~deb12u1 [95,5 kB]

And more tried to fix it this way.

sudo mv var/lib/apt/lists /var/lib/apt/lists.ORIG

sudo apt-get update

and with this commands.

sudo apt-get --allow-releaseinfo-change update

sudo apt-get --allow-releaseinfo-change-suite update

And then I discovered the exact cause of the error...

Unfortunately, the bug exists! Today I tried the mirror switch again, and just
as before, the contents of the sources.list file, the bookworm-security line,
will corrupted.