Upgrade to Ubuntu 14.04 not possible

[Ulldemolins]

I try to upgrade Ubuntu 12.04LTS to Ubuntu 14.04LTS, like it is suggested in the "Gestionnaire des mises à jours"
When arriving to "calculate" the following message apperas:
"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

Si rien de tout cela ne s'applique, veuillez signaler ce bogue en utilisant la commande 'ubuntu-bug ubuntu-release-upgrader-core' dans un terminal."

I use Cinnamon desktop.

Thanks for your help

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.13.0-36.63~precise1-generic 3.13.11.6
Uname: Linux 3.13.0-36-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.7
Architecture: amd64
Date: Thu Sep 11 19:05:54 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1410454591
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 589
 com.ubuntu.update-manager window-width 641
InstallationMedia: Ubuntu 12.04.3 LTS "Precise Pangolin" - Release amd64 (20130820.1)
MarkForUpload: True
PackageArchitecture: all
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 2014-09-11 (0 days ago)
VarLogDistupgradeTermlog: