Unable to upgrade from 12.04 to 14.04 (server)

[Freeben666]

Here is the end of the output from the "do-release-upgrade" command :

Calcul des modifications en cours

Impossible d'évaluer la mise à niveau

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager (not installed)
ProcVersionSignature: Ubuntu 3.2.0-69.103-generic 3.2.62
Uname: Linux 3.2.0-69-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.7
Architecture: amd64
Date: Wed Oct 22 14:40:37 2014
MarkForUpload: True
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-10-22 (0 days ago)

[Freeben666 (freeben666)]

After investigating a little bit more, it appears it is caused by the inability of the upgrade script to upgrade PostgreSQL

From main.log :
2014-10-22 14:40:05,871 DEBUG blacklist expr '^postgresql-.*[0-9]\.[0-9].*' matches 'postgresql-9.1-pgmemcache'
2014-10-22 14:40:05,871 DEBUG The package 'postgresql-9.1-pgmemcache' is marked for removal but it's in the removal blacklist
2014-10-22 14:40:05,885 ERROR Dist-upgrade failed: 'Le paquet « postgresql-9.1-pgmemcache » est marqué pour suppression mais il est dans la liste noire des suppressions.'