I just can't do a release upgrade. No matter what I try.

I'm using ubuntu 12.04 LTS and trying to upgrade to Ubuntu 14.04 LTS.

I have all upgrades for my current version, but when I try to upgrade it returns:

    Checking package manager
    Reading package lists... Done
    Building dependency tree
    Reading state information... Done
    Building data structures... Done

    Calculating the changes

    Calculating the changes

    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.

So, here I am. I already try to remove all ppa's that I added. And a few other things.

Any advice or response for this bug?

Thanks in advance

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.18
ProcVersionSignature: Ubuntu 3.13.0-51.84~precise1-generic 3.13.11-ckt18
Uname: Linux 3.13.0-51-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.8
Architecture: amd64
Date: Fri May 1 16:54:00 2015
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1430509684
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 658
 com.ubuntu.update-manager window-width 617
InstallationMedia: Ubuntu 12.04.3 LTS "Precise Pangolin" - Release amd64 (20130820.1)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2015-05-01 (0 days ago)