ubuntu upgrade to 16 gives 'could not calculate upgrade' error at the step 'setting up new software channels'

[Pinar Demetci]

While upgrading to Ubuntu 16.04.1, at the step "setting up new software channels', I received an error saying 'could not calculate upgrade'. It gave 3 possible reasons for this, 2 of which I remember: -upgrading a pre-release version, -running a current pre-release version, and a third one I don't remember.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-87.133-generic 3.13.11-ckt39
Uname: Linux 3.13.0-87-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.21
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sat Aug 13 15:21:12 2016
InstallationDate: Installed on 2015-01-27 (563 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-08-13 (0 days ago)
VarLogDistupgradeTermlog: