Cannot upgrade to 16.04 from 15.10

[Emmanuel Stapf [ES]]

I cannot upgrade to 16.04. I'm getting:

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.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: ubuntu-release-upgrader-core 1:15.10.14.4
ProcVersionSignature: Ubuntu 4.2.0-36.41-generic 4.2.8-ckt8
Uname: Linux 4.2.0-36-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
Date: Tue May 10 06:16:08 2016
InstallationDate: Installed on 2012-05-15 (1455 days ago)
InstallationMedia: Ubuntu-Server 12.04 LTS "Precise Pangolin" - Release amd64 (20120424.1)
PackageArchitecture: all
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C
 SHELL=/usr/bin/tcsh
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to wily on 2016-05-10 (0 days ago)
VarLogDistupgradeAptclonesystemstate.tar.gz:
 Error: command ['pkexec', 'cat', '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'] failed with exit code 127: polkit-agent-helper-1: pam_authenticate failed: Authentication failure
 Error executing command as another user: Not authorized

 This incident has been reported.