upgrade release ubuntu 14.04

[tranchant]

A message appeared when a release of Ubuntu 14.04 was being processed:

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 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.2.0-67.101-generic-pae 3.2.60
Uname: Linux 3.2.0-67-generic-pae i686
NonfreeKernelModules: fglrx
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: i386
CheckboxSubmission: b010d17896594ed1c5e258ec71e84f62
CheckboxSystem: 2a6f54df59af338184485e85cbcf0d32
Date: Sat Aug 16 13:23:40 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1406975450
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 656
 com.ubuntu.update-manager window-width 985
InstallationMedia: Ubuntu-Netbook 10.04 "Lucid Lynx" - Alpha i386 (20100119)
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 2014-08-16 (0 days ago)
VarLogDistupgradeTermlog: