upgrading from 12.04 LTS to 14.04 LTS crashes

[carmen arenal]

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 nada de esto aplica, informe de este error usando la orden «ubuntu-bug ubuntu-release-upgrader-core» en una terminal.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.13
ProcVersionSignature: Ubuntu 3.2.0-58.88-generic 3.2.53
Uname: Linux 3.2.0-58-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Sat May 10 14:27:51 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1399722806
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 539
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=es_ES.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-05-10 (0 days ago)
VarLogDistupgradeTermlog: