Upgrade from 15.10 to 16.04 LTS is not possible

[Carsten]

when trying to upgrade to wily werewolf, I received the following error message:
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

I already uninstalled all packages I had installed, but still I receive this error message.

Is there another way to upgrade to 16.04?

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: ubuntu-release-upgrader-core 1:15.10.14.4
ProcVersionSignature: Ubuntu 4.2.0-42.49-generic 4.2.8-ckt12
Uname: Linux 4.2.0-42-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sat Jul 30 00:12:08 2016
InstallationDate: Installed on 2016-03-09 (142 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to wily on 2016-07-29 (0 days ago)
VarLogDistupgradeTermlog:

[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!