upgrade to 20.04 from 18.04 unsuccessful

[Kamikaze Blitzkrieg]

(1)Description: Ubuntu 18.04.5 LTS
Release: 18.04

(2)gnome:
  Installed: (none)
  Candidate: 1:3.22+9
  Version table:
     1:3.22+9 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages

(3) I expected the upgrade to 20.04 from 18.04 to happen without any issues.
(4) The upgrade failed with this error message. I have also attached the contents of /var/log/dist-upgrade/apt.log and /var/log/dist-upgrade/main.log

An unresolvable problem occurred while calculating the upgrade.

 This was likely caused by:
 * Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If you want to investigate this yourself the log files in '/var/log/dist-upgrade' will contain details about the upgrade. Specifically, look at 'main.log' and 'apt.log'.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.44
ProcVersionSignature: Ubuntu 4.15.0-149.153-generic 4.15.18
Uname: Linux 4.15.0-149-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.24
Architecture: amd64
CrashDB: ubuntu
Date: Thu Jul 1 16:27:44 2021
InstallationDate: Installed on 2019-06-27 (734 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_IN
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-07-01 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2021-03-30T08:40:06.677120

[Chris Guiver (guiverc)]

Thank you for taking the time to report this bug and helping to make Ubuntu better.

I see a number of 3rd party packages, esp. from https://launchpad.net/~oibaf/+ppa-packages

For best results with a release-upgrade, you should remove all 3rd party packages (bringing your system back to a pure Ubuntu one, ie. no 3rd party packages) then perform the upgrade. The tool ppa-purge can assist with this.

Likely related to https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1069133

(following from https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1922546/comments/2)

[The use of oibaf ppa] 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 ppa:oibaf/graphics-drivers'. After that you can try upgrading again and if it still fails open a new bug report. Thanks and good luck!