Cannot calculate upgrade from 14.04 to 16.04

[Arthur Azevedo de Amorim]

When I try to upgrade from 14.04 to 16.04, I get an error message saying that it "cannot calculate the upgrade". My situation is similar to the one reported at http://askubuntu.com/questions/804875/could-not-calculate-upgrade-14-04-16-04: inspecting the log, I find that about 800 packages are marked as "broken" on my system. Given that many of them seem to be quite fundamental, I feel uneasy about simply deleting everything and trying to reinstall it.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-92.139-generic 3.13.11-ckt39
Uname: Linux 3.13.0-92-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.21
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Tue Aug 9 11:33:42 2016
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-precise-amd64-20120703-2
InstallationDate: Installed on 2013-03-23 (1235 days ago)
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20120703-15:08
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-08-09 (0 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[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 package version numbers greater than the release to which you are upgrading. You can revert the PPA by installing the ppa-purge package from the official Ubuntu archive and then running 'sudo ppa-purge oibaf'. After that you try upgrading and again and if it still fails open a new bug report. Thanks and good luck!

[Arthur Azevedo de Amorim (arthur-aa)]

Thanks for your reply. I understand that I need to disable some PPA, but it seems that oibaf doesn't do the trick. When I run 'sudo ppa-purge oibaf', I get the following output:

Updating packages lists
W: There is no public key available for the following key IDs:
1397BC53640DB551
PPA to be removed: oibaf ppa
Warning: Could not find package list for PPA: oibaf ppa

Is there any other PPA that I should try to disable?