upgrade from 14.4 to 15.4 failed

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Es konnte nicht ermittelt werden, welche Systemaktualisierungen verfügbar sind

Ein unlösbares Problem trat beim Ermitteln der Systemaktualisierung auf.

 Dies kann verursacht worden sein durch:
 * Aktualisieren auf eine Vorabversion von Ubuntu
 * Verwendung der aktuellen Vorabversion von Ubuntu
 * Inoffizielle Softwarepakete, die nicht von Ubuntu bereitgestellt wurden

Falls nichts von dem zutrifft, melden Sie diesen Fehler bitte, indem Sie den Befehl »ubuntu-bug ubuntu-release-upgrader-core« in einem Terminal ausführen.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-83.127-generic 3.13.11-ckt35
Uname: Linux 3.13.0-83-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Wed Mar 16 21:30:54 2016
InstallationDate: Installed on 2015-03-02 (380 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 2016-03-16 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update.manager.release.upgrades: 2016-03-14T22:32:41.431356

[Ubuntu Foundations Team Bug Bot (crichton)]

Reviewing the files from your upgrade attempt you seem to have enabled the xorg-edgers-ppa. 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. Thanks.

[This is an automated message. I apologize if it reached you inappropriately; please just reply to this message indicating so.]

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug 1069133, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Please continue to report any other bugs you may find.
[This is an automated message. I apologize if it reached you inappropriately; please just reply to this message indicating so.]