upgrader from xenial to bionic fails without any clear error message

[Claviger]

TRying to upgrade from xenial to bionic on headless server. The upgrader fails without a clear reason why

Description: Ubuntu 16.04.6 LTS
Release: 16.04

apt-cache policy update-manager-core
update-manager-core:
  Installed: 1:16.04.16
  Candidate: 1:16.04.16
  Version table:
 *** 1:16.04.16 900
        900 http://us.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        900 http://us.archive.ubuntu.com/ubuntu xenial-updates/main i386 Packages
        100 /var/lib/dpkg/status
     1:16.04.12 900
        900 http://us.archive.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        900 http://us.archive.ubuntu.com/ubuntu xenial-security/main i386 Packages
     1:16.04.3 900
        900 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        900 http://us.archive.ubuntu.com/ubuntu xenial/main i386 Packages

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.27
ProcVersionSignature: Ubuntu 4.4.0-167.196-generic 4.4.197
Uname: Linux 4.4.0-167-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.19
Architecture: amd64
CrashDB: ubuntu
Date: Fri Oct 25 16:01:51 2019
InstallationDate: Installed on 2017-03-03 (966 days ago)
InstallationMedia: Ubuntu-Server 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.8)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-10-25 (0 days ago)

[Claviger (claviger-pc)]

apt pinning was in use in /etc/apt/preferences