can't upgrade from 14.04 to 15.04

[Alex Volkov]

dist upgrade error message:
"Не удалось рассчитать обновление системы

Обнаружена неразрешимая проблема при расчёте обновления.

 Это может быть вызвано тем, что:
 * Обновление до пре-релиза Ubuntu
 * Использование текущей пре-релиз версии Ubuntu
 * Неофициальное ПО, не предоставляемое Ubuntu

Если ничего из перечисленного не подходит, сообщите об этой ошибке, выполнив команду «ubuntu-bug ubuntu-release-upgrader-core» в терминале."

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-67.109-generic 3.13.11-ckt27
Uname: Linux 3.13.0-67-generic i686
ApportVersion: 2.14.1-0ubuntu3.18
Architecture: i386
CrashDB: ubuntu
CurrentDesktop: GNOME
Date: Tue Oct 27 19:47:50 2015
InstallationDate: Installed on 2013-10-20 (736 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2015-10-27 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update.manager.release.upgrades: 2015-10-26T20:17:58.727376

[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.