release upgrade to 14.04 stops

[Andrea Cometa]

when upgrading from 12.04.5 to 14.04, ubuntu stops
first message:
Your graphics hardware may not be fully supported in Ubuntu 14.04.

L'esecuzione dell'ambiente desktop «unity» non è completamente
supportata dal proprio hardware. Dopo l'aggiornamento ci si troverà
in un ambiente molto rallentato. Si consiglia di mantenere la
versione LTS per il momento. Per ulteriori informazioni consultare la
pagina https://wiki.ubuntu.com/X/Bugs/UpdateManagerWarningForUnity3D.
Continuare comunque con l'aggiornamento?

then
Impossibile calcolare l'avanzamento

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.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.18
ProcVersionSignature: Ubuntu 3.2.0-77.112-generic 3.2.66
Uname: Linux 3.2.0-77-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.8
Architecture: amd64
Date: Sun Mar 8 13:54:20 2015
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
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 2015-03-08 (0 days ago)

[Brian Murray (brian-murray)]

Reviewing the files from your upgrade attempt you seem to have enabled a PPA that provides Xorg packages. This will prevent the upgrade as that PPA contains different versions of packages essential to Ubuntu. The page, https://launchpad.net/~xorg-edgers/+archive/ppa, contains details on to how to revert to the official Xorg packages for the xorg-edgers PPA. Instructions for reverting to official Xorg packages should be similar for the PPA that you have enabled.