error upgrading from 12.04 to 14.04 server

[Mario Di Ture]

...
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

Se non vengono installati, segnalare il problema usando il comando
«ubuntu-bug ubuntu-release-upgrader-core» in un terminale.

Ripristino dello stato originale del sistema

Interruzione
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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 x86_64
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Mon Aug 18 13:00:31 2014
GsettingsChanges:

InstallationMedia: Ubuntu-Server 11.04 "Natty Narwhal" - Release amd64 (20110426)
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 2014-08-18 (0 days ago)