Can't upgrade fro Ubuntu 12.04.05 LTS to 14.04.1 LTS though I disabled all external repositories in sources.list and deleted sources.list.d folder

[Kunal Shah]

I have been trying constantly and failing each time when I try to upgrade fro Ubuntu 12.04.05 LTS to 14.04.1 LTS.
I disabled all external repositories in sources.list and deleted sources.list.d folder so I don't see any error in the upgrade manager but the upgrade doesn't work.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.13.0-39.66~precise1-generic 3.13.11.8
Uname: Linux 3.13.0-39-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.7
Architecture: amd64
Date: Mon Nov 3 07:27:54 2014
GsettingsChanges:

InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
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 2014-11-03 (0 days ago)
VarLogDistupgradeTermlog:

[Kunal Shah (shahkunal9)]

This is the error I receive in step 2 (Setting new software channels):

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.