Upgrade to Ubuntu 20.04 fails because of PPAs. How to proceed?

Question

I'm on Ubuntu 19.10 and I'm trying to upgrade to 20.04. Unfortunately the upgrade failed with a message saying that it "possibly failed because of unofficial software packages which are not provided by Ubuntu" (translated from Dutch).

The notification suggests to use ppa-purge "to remove the software from a Launchpad PPA". It doesn't specify which PPA, so I checked the log file /var/log/dist-upgrade/20200528-1225/main.log. In there I don't see any problems though. Here's a paste of the full main.log file. I pasted the end of the log file below.

Does anybody know how I can solve this so that I can upgrade to 20.04?

2019-10-30 16:00:15,624 DEBUG Start checking for obsolete pkgs
2019-10-30 16:00:21,655 DEBUG package 'multiarch-support' produces an unwanted removal 'nextcloud-client', skipping
2019-10-30 16:00:22,705 DEBUG 'multiarch-support' scheduled for remove but not safe to remove, skipping
2019-10-30 16:00:26,392 DEBUG package 'libgnome-keyring0' produces an unwanted removal 'nextcloud-client', skipping
2019-10-30 16:00:27,797 DEBUG 'libgnome-keyring0' scheduled for remove but not safe to remove, skipping
2019-10-30 16:00:28,098 DEBUG package 'libjson-c3' produces an unwanted removal 'libisccfg160', skipping
2019-10-30 16:00:29,442 DEBUG 'libjson-c3' scheduled for remove but not safe to remove, skipping
2019-10-30 16:00:31,026 DEBUG package 'libgnome-keyring-common' produces an unwanted removal 'nextcloud-client', skipping
2019-10-30 16:00:32,251 DEBUG 'libgnome-keyring-common' scheduled for remove but not safe to remove, skipping
2019-10-30 16:00:33,215 DEBUG Finish checking for obsolete pkgs
2019-10-30 16:00:33,273 DEBUG The following packages are marked for removal: libx265-165 linux-modules-4.15.0-1056-oem liblvm2cmd2.02 libglade2-0 libjpeg-turbo-progs gir1.2-mutter-4 ifupdown libparse-debianchangelog-perl libapt-inst2.0 libobjc-8-dev libllvm8 gir1.2-clutter-1.0 libcrystalhd3 openjfx-source libgupnp-1.0-4 ubuntu-wallpapers-cosmic libgtksourceview-3.0-common libip6tc0 pppoeconf libmutter-4-0 example-content command-not-found-data libapt-pkg5.0 ubuntu-wallpapers-disco libisl19 libvpx5 linux-oem-headers-4.15.0-1056 gir1.2-clutter-gst-3.0 gnome-calculator gir1.2-cogl-1.0 libgnome-desktop-3-17 g++-8 gir1.2-gtksource-3.0 xscreensaver-data python3-evdev liblouisutdml8 liblvm2app2.2 chromium-codecs-ffmpeg-extra linux-headers-4.15.0-1056-oem libgssdp-1.0-3 linux-image-4.15.0-1056-oem libigdgmm5 libmysqlclient20 libgtksourceview-3.0-1 libhtml-template-perl gir1.2-coglpango-1.0 libebook-contacts-1.2-2 libphobos2-ldc-shared82 libtagc0 libgeos-3.7.1 pppconfig libecal-1.2-19 libpoppler85 libip4tc0 openjfx libgtkspell3-3-0 gir1.2-gtkclutter-1.0 libreoffice-avmedia-backend-gstreamer libdouble-conversion1
2019-10-30 16:01:14,154 INFO cache.commit()
2019-10-30 16:01:34,131 DEBUG quirks: running PostCleanup
2019-10-30 16:01:34,131 DEBUG running Quirks.PostCleanup
2019-10-30 16:01:34,132 DEBUG Running PostInstallScript: './xorg_fix_proprietary.py'

Answer 1

Ok, I found this answer, which suggests to search the logs for a line like this:

2013-10-17 15:00:30,543 ERROR Dist-upgrade failed: 'The package 'xubuntu-desktop' is marked for removal but it is in the removal blacklist.'

For future readers, this is what I did. I ran a search like this:

sudo apt install ack-grep
ack "ERROR Dist-upgrade failed" /var/log/dist-upgrade/

which gave this output:

ack: apt-clone_system_state.tar.gz: cannot open file for reading
main.log 
115:2020-05-28 13:59:48,248 ERROR Dist-upgrade failed: 'The package 'postgresql-12-postgis-3' is marked for removal but it is in the removal blacklist.'

20200528-1350/main.log 
117:2020-05-28 13:48:27,555 ERROR Dist-upgrade failed: 'The package 'postgresql-11-postgis-2.5' is marked for removal but it is in the removal blacklist.'

I then uninstalled those packages:

sudo apt purge postgresql-12-postgis-3
sudo apt purge postgresql-11-postgis-2.5

and after that the upgrade is now continuing to install..