Comment 2 for bug 1876486

This is something to do with linux 5 kernel api

You need to manually downgrade systemd - well - that's what I did.

when you boot with grub or refind - edit default entry and change the init=/bin/init to init=/bin/bash

That will bypass systemd starting up and drop you straight into a shell.

mount -o remount /
mount /usr
mount /var
mount /dev/sdc1 /mnt
dpkg -i /mnt/systemd_241-7~deb10u3_amd64.deb

On a system that works or download a previous version somehow.

dpkg -l systemd
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-==============-============-============-=================================
ii systemd 245.5-2 amd64 system and service manager
root@leet:/home/wozza# apt-cache policy systemd
systemd:
  Installed: 245.5-2
  Candidate: 245.5-2
  Version table:
 *** 245.5-2 500
        500 http://ftp.iinet.net.au/debian/debian bullseye/main amd64 Packages
        100 /var/lib/dpkg/status
     241-7~deb10u3 500
        500 http://ftp.iinet.net.au/debian/debian stable/main amd64 Packages
root@leet:/home/wozza# apt-get download systemd=241-7~deb10u3
Get:1 http://ftp.iinet.net.au/debian/debian stable/main amd64 systemd amd64 241-7~deb10u3 [3,495 kB]
Fetched 3,495 kB in 11s (321 kB/s)
W: Download is performed unsandboxed as root as file '/home/wozza/systemd_241-7~deb10u3_amd64.deb' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
root@leet:/home/wozza# ll systemd_241-7~deb10u3_amd64.deb
-rw-r--r-- 1 root root 3495428 Jan 31 15:11 systemd_241-7~deb10u3_amd64.deb
root@leet:/home/wozza# mount /dev/sdc1 /mnt
root@leet:/home/wozza# cp systemd_241-7~deb10u3_amd64.deb /mnt/
root@leet:/home/wozza# cd /mnt/
root@leet:/mnt# mkdir unpack
root@leet:/mnt# cd unpack/
root@leet:/mnt/unpack# dpkg-deb -R ../systemd_241-7~deb10u3_amd64.deb .

worst comes to worse can copy the files across from unpack directory - good luck.