i cannot do the upgrade 12.04.4 a 14.04.1

[Stefano Tarquini]

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 3.2.60
Uname: Linux 3.2.0-67-generic i686
NonfreeKernelModules: wl
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: i386
Date: Wed Aug 20 22:10:46 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1407002247
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 527
 com.ubuntu.update-manager window-width 600
InstallationMedia: Xubuntu 11.10 "Oneiric Ocelot" - Release i386 (20111012)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=it:en
 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 2014-08-20 (0 days ago)
VarLogDistupgradeTermlog:

[Stefano Tarquini (esteban-turquoise)]

i uninsttalled the plugin whisker and its repository but i have the same error