dist-upgrade to 14.04 failed

[Daniele]

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

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

Even after
sudo apt-get clean
sudo apt-get autoremove
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

the problem still appears-

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.5.0-54.81~precise1-generic 3.5.7.33
Uname: Linux 3.5.0-54-generic i686
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: i386
Date: Mon Aug 25 23:34:26 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1409002404
 com.ubuntu.update-manager window-height 500
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04 "Precise" - Build i386 LIVE Binary 20130822-17:25
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=it_IT.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-08-25 (0 days ago)
VarLogDistupgradeTermlog:

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.