Upgrade from 13.10 to 14.04 impossible

[Mérôme Jardin]

- unable to upgrade to ubuntu 14_04 :
- I tried today to upgrade from Ubuntu 13.10 to 14.04
crash message :

"Impossible d'évaluer la mise à niveau (= unable to evaluate the upgrade)

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."

The message is the same if I try to upgrade with the terminal. It is the same when I disable the unofficial software packages sources or the non-Ubuntu ones.

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: ubuntu-release-upgrader-core 1:0.205.7
ProcVersionSignature: Ubuntu 3.11.0-26.45-generic 3.11.10.12
Uname: Linux 3.11.0-26-generic x86_64
ApportVersion: 2.12.5-0ubuntu2.2
Architecture: amd64
CrashDB: ubuntu
Date: Fri Aug 8 13:40:05 2014
InstallationDate: Installed on 2014-03-01 (159 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to saucy on 2014-08-08 (0 days ago)