Server running Ubuntu is suddenly Debian bullseye?

One of our servers is supposed to be running Ubuntu 16.04 LTS, but somehow Debian bullseye got into the mix. None of my colleages seems to know how this happened, and I would like to get it back on track... I hope that it is just a matter of mis-configuration somewhere, that can be corrected.

less /etc/os-release
Code:PRETTY_NAME="Debian GNU/Linux bullseye/sid"
NAME="Debian GNU/Linux"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"uname -a
Code:Linux xxxhostnamexxx 4.4.0-169-generic #198-Ubuntu SMP Tue Nov 12 10:38:00 UTC 2019 x86_64 GNU/Linuxlsb_release -a
Code:No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu GNU/Linux testing/unstable
Release: testing/unstable
Codename: n/aless /etc/apt/sources.list
Code:deb http://dk.archive.ubuntu.com/ubuntu/ xenial main restricted
deb-src http://dk.archive.ubuntu.com/ubuntu/ xenial main restricted
deb http://dk.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
deb-src http://dk.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
deb http://dk.archive.ubuntu.com/ubuntu/ xenial universe
deb-src http://dk.archive.ubuntu.com/ubuntu/ xenial universe
deb http://dk.archive.ubuntu.com/ubuntu/ xenial-updates universe
deb-src http://dk.archive.ubuntu.com/ubuntu/ xenial-updates universe
deb http://dk.archive.ubuntu.com/ubuntu/ xenial multiverse
deb-src http://dk.archive.ubuntu.com/ubuntu/ xenial multiverse
deb http://dk.archive.ubuntu.com/ubuntu/ xenial-updates multiverse
deb-src http://dk.archive.ubuntu.com/ubuntu/ xenial-updates multiverse
deb http://dk.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://dk.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu xenial-security main restricted
deb-src http://security.ubuntu.com/ubuntu xenial-security main restricted
deb http://security.ubuntu.com/ubuntu xenial-security universe
deb-src http://security.ubuntu.com/ubuntu xenial-security universe
deb http://security.ubuntu.com/ubuntu xenial-security multiverse
deb-src http://security.ubuntu.com/ubuntu xenial-security multiversePackage origin
I also made a small script to investigate the origin of all installed packages. The script basically looks at the output from `apt policy [packagename]`. And it seems that all packages originates from xenial.

So how dit bullseye get into os-release? And why do lsb_release not recognize the OS as Ubuntu Xenial? Any advice is greatly appreciated.

UPDATE:

If I try to add a repository, if fails with this error:

Code:aptsources.distro.NoDistroTemplateException: Error: could not find a distribution template for Ubuntu/n/a