Upgrading from 12.04.5 LTS to 14.04.1 LTS reports unresolvable issues

[Corben]

Tried today to upgrade from 12.04.5 LTS to 14.04.1 LTS.
First try resulted in errors with ubuntu-desktop package, which wasn't installed. I installed it manually, and now this error message occurs:

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

Cannot upgrade from 12.04 to 14.04 now.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.2.0-75.110-generic 3.2.64
Uname: Linux 3.2.0-75-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu17.8
Architecture: amd64
Date: Tue Jan 27 11:42:43 2015
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
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 2015-01-27 (0 days ago)

[Corben (corben-dallas)]

Found a solution to my problem.
A package from ppa:irie/blender caused the problems. It installed a python3 version, which couldn't be upgraded. After uninstalling this package, the upgrade went through.

[Brian Murray (brian-murray)]

Right, that PPA provides a version number greater than the one in Trusty and that's why the release-upgrader can't sort out the upgrade path.

[oroboros (oroboros-n)]

Having the same problems, but the cause is something unspecified.

I thought there was a package from some ppa causing the problems, and I picked the Enlightenment package as the culprit. Deselected it in the update manager, but that didn't solve the upgrade problem.

[oroboros (oroboros-n)]

The untrusted packages from non-authenticated sources:

e17 e17-data evas-loaders libedbus1

When I deselect this package, it causes a chain reaction of sorts that prevents the upgrade from 12.04 to 14.04 going smoothly. :(

[oroboros (oroboros-n)]

Just now tried it again, and the dialog box popped back up:

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.