[SOLVED] LTS dist upgrade failure from 16.04 to 18.04

[Thibaut]

I just upgrade from 14.04 to 16.04 before. I had some broken packages that I corrected.

Then:
sudo apt-get update
sudo apt-get upgrade
sudo do-release-upgrade

leads me to:
-> Un problème insoluble est survenu lors de la préparation de la mise à
niveau.
-> ENGLISH: An unresolvable problem occurred while calculating the upgrade

In /var/log/dist-upgrade/XXX/main.log :

-> ERROR Dist-upgrade failed: 'E:Erreur, pkgProblem::Resolve a généré des ruptures
-> ENGLISH: pkgProblemResolver::Resolve generated breaks, this may be caused by held packages

Thanks for any help

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.4.0-141.167-generic 4.4.162
Uname: Linux 4.4.0-141-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri Jan 4 00:01:45 2019
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-01-03 (0 days ago)
mtime.conffile..etc.update-manager.release-upgrades: 2019-01-02T16:24:36.364160

[Thibaut (zaraf59)]

Figured out that I just had to install some packages (libgfortran4 and cpp-7 for my case) required by the hold back packages (listed in Apt log)..

.. and finally the Bionic Beaver could be brought back to life.

I also did this before because I ran through the same error: https://askubuntu.com/questions/943463/e-problem-executing-scripts-apt-updatepost-invoke-success-error-during-apt-ge

.. but not sure if it is related.