ubuntu-release-upgrader craches on " Setting new software channels" !

[BenginM]

on my Attempt to upgrade from trusty gnome desktop version to xenial xerus using the upgrader GUI, it craches on the 2nd step ' setting new software channels' with the error :

" 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 ".

##

sm@tru:~$ lsb_release -rd
Description: Ubuntu 14.04.3 LTS
Release: 14.04
sm@tru:~$ apt-cache policy ubuntu-release-upgrader-core
ubuntu-release-upgrader-core:
  Installed: 1:0.220.8
  Candidate: 1:16.04.3
  Version table:
     1:16.04.3 0
        500 http://sa.archive.ubuntu.com/ubuntu/ xenial/main amd64 Packages
 *** 1:0.220.8 0
        100 /var/lib/dpkg/status
sm@tru:~$

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.19.0-47.53~14.04.1-generic 3.19.8-ckt10
Uname: Linux 3.19.0-47-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: GNOME
Date: Sat Jan 23 04:19:06 2016
InstallationDate: Installed on 2016-01-21 (1 days ago)
InstallationMedia: Ubuntu-GNOME 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-01-23 (0 days ago)
VarLogDistupgradeTermlog: