Unable to upgrade from 14.04 to 16.04

[Pavel]

Unable to upgrade from 14.04 to 16.04

Calculating the changes

Could not calculate the upgrade

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.

Restoring original system state

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-92.139-generic 3.13.11-ckt39
Uname: Linux 3.13.0-92-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.21
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sun Jul 31 12:07:20 2016
InstallationDate: Installed on 2015-02-08 (538 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-07-31 (0 days ago)

[Launchpad Janitor (janitor)]

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

[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 package version numbers greater than the release to which you are upgrading. You can revert the PPA by installing the ppa-purge package from the official Ubuntu archive and then running 'sudo ppa-purge oibaf'. After that you try upgrading and again and if it still fails open a new bug report. Thanks and good luck!