Can’t upgrade from 14.04 to 16.04 with PPAs enabled

[flrtx]

Follow-up from Bug #1824680

I am unable to start the upgrade process from 14.04 to 16.04.

"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 have never used a pre-release of Ubuntu but I may have installed packages not provided by Ubuntu. I've got some PPAs activated, mainly to get a more recent version of Ardour and Jack.

How can I resolve this problem and upgrade to the next release? Is there a way around installing from scratch?

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.10
ProcVersionSignature: Ubuntu 3.13.0-168.218-generic 3.13.11-ckt39
Uname: Linux 3.13.0-168-generic i686
NonfreeKernelModules: wl
ApportVersion: 2.14.1-0ubuntu3.29
Architecture: i386
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri Apr 19 20:42:24 2019
InstallationDate: Installed on 2012-05-19 (2526 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2019-04-15 (3 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

You'll want to use the tool ppa-purge from the package ppa-purge to remove the PPAs you have enabled and revert back to the official versions of the package which the PPAs provide. Here's an example from the man page.

        sudo ppa-purge -o sarvatt -p xorg-testing
        will remove https://launchpad.net/~sarvatt/+archive/xorg-testing

[flrtx (fltrx)]

I did my best to purge all PPAs. However, I got 'Warning: Could not find package list for PPA:' on a couple of them. I tried dist-upgrading nevertheless and it didn’t work. Any suggestions?

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]