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:amd6
Public bug reported:
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,
pkgProb
