Upgrade from Ubuntu Mate 18.04 to 20.04 fails

[Ingo Wilhelm]

An upgrade from Ubuntu Mate 18.04 to 20.04 fails.

/var/log/dist-upgrade/main.log shows :

2020-05-01 14:53:13,658 DEBUG need_server_mode(): run in 'desktop' mode, (because of key deps for 'ubuntu-desktop')
2020-05-01 14:58:12,251 ERROR Dist-upgrade failed: 'E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.'

Running
apt remove ubuntu-desktop
results in
Package 'ubuntu-desktop' is not installed, so not removed

So ubuntu-desktop is not installed ...

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 4.15.0-99.100-generic 4.15.18
Uname: Linux 4.15.0-99-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
CrashDB: ubuntu
Date: Fri May 1 15:02:56 2020
InstallationDate: Installed on 2015-06-14 (1782 days ago)
InstallationMedia: Ubuntu MATE 14.04.1 "Trusty Tahr" - final amd64 (20141111)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-05-01 (0 days ago)
VarLogDistupgradeTermlog:

[Ingo Wilhelm (ingo-wilhelm)]

I found the problem.

/var/log/dist-upgrade/apt.log
contained the lines

Broken libk3b7-extracodecs:amd64 Depends on libavcodec58:amd64 < none | 7:4.2.2-1ubuntu1 @un uH > (>= 7:4.0)
  Considering libavcodec58:amd64 70 as a solution to libk3b7-extracodecs:amd64 1
  MarkKeep libk3b7-extracodecs:amd64 < 17.12.3-0ubuntu3 -> 19.12.3-0ubuntu1 @ii umU Ib > FU=0
  Holding Back libk3b7-extracodecs:amd64 rather than change libavcodec58:amd64

I eventually found out that libk3b7-extracodecs belongs to the DVD burning application K3B.

After deinstalling K3B with

apt remove k3b
apt autoremove

the upgrade to Ubuntu Mate 20.04 was successful.