Could not calculate the upgrade

[Iman]

Existing version: 14.04
Trying to upgrade to Version 15.10, I got this message during the upgrade procedure:

Distribution Upgrade => Setting new software channels
<
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.
>

None of the three proposed causes being true, it brings me here to report the bug.
I'd appreciate it if you shed some light on the case and help me through the installation.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.16.0-71.92~14.04.1-generic 3.16.7-ckt27
Uname: Linux 3.16.0-71-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.21
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sun Jun 5 17:09:16 2016
InstallationDate: Installed on 2015-12-29 (158 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 2016-06-05 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update.manager.release.upgrades: 2016-06-05T16:20:10.901445

[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!