Update 12.04 LTS to 14.04 LTS: config new package sources

[LaunchpadUser]

When upgrading from 12.04 LTS to 14.04 LTS, the following error occured:

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Es konnte nicht ermittelt werden, welche Systemaktualisierungen verfügbar sind

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

Falls nichts von dem zutrifft, melden Sie diesen Fehler bitte, indem Sie den Befehl »ubuntu-bug ubuntu-release-upgrader-core« in einem Terminal ausführen.
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I deactivated all foreign packages (PPAs) before starting the upgrade.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.13
ProcVersionSignature: Ubuntu 3.2.0-60.91-generic 3.2.55
Uname: Linux 3.2.0-60-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
CheckboxSubmission: baefda4a6a93cd1bb149d575738515c5
CheckboxSystem: daed2f3d6643b4a84b4520a2427f8c2b
Date: Fri Apr 18 12:02:44 2014
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-04-18 (0 days ago)
VarLogDistupgradeTermlog: