Unable to upgrade to ubuntu 16.04

[azure32]

After trying to upgrade to ubuntu 16.04 (from 15.10), I got the error message that

"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."

I did a sudo apt-get clean, update, dist-upgrade, etc.

I checked the log with $ grep ERROR /var/log/dist-upgrade/main.log :
2016-05-12 15:00:56,655 ERROR Dist-upgrade failed: 'E:Unable to correct problems, you have held broken packages.'

However, $ sudo apt-get -f update does nothing to fix the problem, and all other methods I could think of show that I have no broken packages.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: ubuntu-release-upgrader-core 1:15.10.14.4
ProcVersionSignature: Ubuntu 3.16.0-52.71~14.04.1-generic 3.16.7-ckt18
Uname: Linux 3.16.0-52-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Thu May 12 15:01:25 2016
InstallationDate: Installed on 2015-07-29 (288 days ago)
InstallationMedia: Ubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to wily on 2016-05-12 (0 days ago)

[Brian Murray (brian-murray)]

Reviewing the files from your upgrade attempt you seem to have enabled a PPA that provides Xorg packages. This will prevent the upgrade as that PPA contains different versions of packages essential to Ubuntu. The page, https://launchpad.net/~xorg-edgers/+archive/ppa, contains details on to how to revert to the official Xorg packages for the xorg-edgers PPA. Instructions for reverting to official Xorg packages should be similar for the PPA that you have enabled.