Failed upgrade from 16.04 to 18.04: could not calculate upgrade

[Stefano Bellotti]

-Source package: ubuntu-release-upgrader-core

1) lsb_release -rd:
Description: Ubuntu 16.04.5 LTS
Release: 16.04

2) apt-cache policy ubuntu-release-upgrader-core:
ubuntu-release-upgrader-core:
  Installato: 1:16.04.25
  Candidato: 1:16.04.25
  Tabella versione:
 *** 1:16.04.25 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main i386 Packages
        500 http://archive.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu xenial-security/main i386 Packages
        100 /var/lib/dpkg/status
     1:16.04.12 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu xenial/main i386 Packages

3) I expected to upgrade smoothly from Ubuntu 16.04 LTS to 18.04 LTS.

4) Instead, when using 'sudo do-release-upgrade' the process stops, throwing the error message 'could not calculate upgrade' and suggesting different causes. Unfortunately, all of them are beyond my understanding and therefore I don't know which package could cause this problem.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.4.0-133.159-generic 4.4.134
Uname: Linux 4.4.0-133-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Aug 20 18:15:32 2018
InstallationDate: Installed on 2014-06-21 (1520 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-08-20 (0 days ago)