Comment 7 for bug 1903838

I was able to reproduce this issue. The problem occurs when safe-rm is installed.

How to reproduce using lxc:

lxc launch images:ubuntu/18.04 bughunt
lxc exec bughunt bash

once you're inside the container run:

apt install ubuntu-release-upgrader-core safe-rm
do-release-upgrade

answer yes so that the upgrade initiates

when the upgrader reaches "Unpacking libc6:amd64 (2.31-0ubuntu9.9)" it fails
with
===============
/usr/bin/perl: error while loading shared libraries: libcrypt.so.1: cannot open shared object file: No such file or directory
dpkg: error while cleaning up:
 rm command for cleanup subprocess returned error exit status 127
/usr/bin/perl: error while loading shared libraries: libcrypt.so.1: cannot open shared object file: No such file or directory
dpkg: error processing archive /var/cache/apt/archives/libcrypt1_1%3a4.4.10-10ubuntu4_amd64.deb (--unpack):
 rm command for cleanup subprocess returned error exit status 127
Errors were encountered while processing:
 /var/cache/apt/archives/libcrypt1_1%3a4.4.10-10ubuntu4_amd64.deb
Exception during pm.DoInstall(): E:Sub-process /usr/bin/dpkg returned an error code (1)
=========

it appears that a quick workaround is to uninstall safe-rm before upgrading and reinstall afterwards

thank you