do-release-upgrade hangs in libc6 preinst while "Checking init scripts..."

[Marius Gedminas]

I've a bunch of these "virtual private servers" (actually from OpenVZ containers) from iv.lt. I've been upgrading them from Ubuntu 14.04 to 16.04. Every time the upgrade hangs in libc6. First, it shows a warning in a nice colourful ncurses dialog

    Kernel version not supported

    This version of the GNU libc requires kernel version 3.2 or later. Older versions might work but are not officially supported.
    Please consider upgrading your kernel.

then it prints

    Checking for services that may need to be restarted...
    Checking init scripts...
    /var/lib/dpkg/tmp.ci/preinst: 355: /var/lib/dpkg/tmp.ci/preinst: runlevel: not found

at the bottom without clearing the screen and hangs.

I can make the upgrade finish if I do

    touch /etc/ld.so.nohwcap
    rm /etc/ld.so.cache /var/cache/ldconfig/aux-cache
    pkill preinst

and then it apparently completes successfully.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:0.220.10
Uname: Linux 2.6.32-042stab128.2 x86_64
ApportVersion: 2.14.1-0ubuntu3.29
Architecture: amd64
CrashDB: ubuntu
Date: Fri Aug 3 15:21:20 2018
PackageArchitecture: all
ProcEnviron:
 LC_CTYPE=lt_LT.UTF-8
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=lt_LT.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: release-upgrade
UpgradeStatus: Upgraded to xenial on 2018-08-03 (0 days ago)
VarLogDistupgradeAptclonesystemstate.tar.gz: Error: command ['pkexec', 'cat', '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'] failed with exit code 127: Error getting authority: Error initializing authority: Error calling StartServiceByName for org.freedesktop.PolicyKit1: GDBus.Error:org.freedesktop.DBus.Error.Spawn.FailedToSetup: Failed to setup environment correctly
VarLogDistupgradeApttermlog: Error: [Errno 13] Permission denied: '/var/log/dist-upgrade/apt-term.log'

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.