Ubuntu – Kubuntu 18.04 no more GUI after upgrade from 17.10

I think I have narrowed this down a bit. By first removing my fglrx proprietary AMD graphics driver, so that Ctl-Alt-F1 would get me a useful terminal, and then by moving the following file aside, I could get from the black screen back to a reasonably healthy desktop:

$HOME/.config/plasma-org.kde.plasma.desktop-appletsrc

Testing could proceed rapidly, because, after each of many tests moving aside various $HOME/.config and $HOME/.cache files, I could quickly see if that got me a black screen, or a reasonable sddm desktop, by invoking the command:

systemctl restart sddm

On the occasions where I got a black screen, I saw signs of a segfault crash in the following log files for kactivitymanage:

/var/log/kern.log
/var/log/syslog
/var/log/apport.log

A typical kern.log or syslog entry was:

May 31 07:58:47 mypc kernel: [ 3123.656160] kactivitymanage[14555]: segfault at 7f93c6ae9cd0 ip 00007f93ace5a031 sp 00007ffcbf253d68 error 4 in libQt5Sql.so.5.4.1[7f93ace46000+3f000]

A typical apport.log entry was:

ERROR: apport (pid 15009) Sun May 31 07:58:47 2015: executable: /usr/bin/kactivitymanagerd (command line "/usr/bin/kactivitymanagerd start-daemon")
ERROR: apport (pid 15009) Sun May 31 07:58:47 2015: apport: report /var/crash/_usr_bin_kactivitymanagerd.1000.crash already exists and unseen, doing nothing to avoid disk usage DoS

Examining my customized (and failing) .config/plasma-org.kde.plasma.desktop-appletsrc, I can see further evidence of the problem. The bad (black) plasma config file is missing the following stuff (no wonder the screen is black - almost nothing is configured ):

[Containments][1]
activityId=
formfactor=2
immutability=1
lastScreen=0
location=4
plugin=org.kde.panel
wallpaperplugin=org.kde.image

[Containments][1][Applets][2]
immutability=1
plugin=org.kde.plasma.kickoff

[Containments][1][Applets][2][Configuration][Shortcuts]
global=Alt+F1

[Containments][1][Applets][2][Shortcuts]
global=Alt+F1

[Containments][1][Applets][3]
immutability=1
plugin=org.kde.plasma.pager

[Containments][1][Applets][4]
immutability=1
plugin=org.kde.plasma.taskmanager

[Containments][1][Applets][5]
immutability=1
plugin=org.kde.plasma.systemtray

[Containments][1][Applets][5][Configuration][Containments][8]
formfactor=2
location=4

[Containments][1][Applets][5][Configuration][Containments][8][Applets][10]
immutability=1
plugin=org.kde.plasma.clipboard

[Containments][1][Applets][5][Configuration][Containments][8][Applets][11]
immutability=1
plugin=org.kde.plasma.devicenotifier

[Containments][1][Applets][5][Configuration][Containments][8][Applets][12]
immutability=1
plugin=org.kde.kdeconnect

[Containments][1][Applets][5][Configuration][Containments][8][Applets][13]
immutability=1
plugin=org.kde.plasma.notifications

[Containments][1][Applets][5][Configuration][Containments][8][Applets][14]
immutability=1
plugin=org.kde.plasma.printmanager

[Containments][1][Applets][5][Configuration][Containments][8][Applets][15]
immutability=1
plugin=org.kde.muonnotifier

[Containments][1][Applets][5][Configuration][Containments][8][Applets][16]
immutability=1
plugin=org.kde.plasma.networkmanagement

[Containments][1][Applets][5][Configuration][Containments][8][Applets][9]
immutability=1
plugin=org.kde.plasma.battery

[Containments][1][Applets][6]
immutability=1
plugin=org.kde.plasma.digitalclock

[Containments][1][General]
AppletOrder=2;3;4;5;6

So ... it would appear that something is writing out a bad plasma config file, with much of the good stuff removed.

This was solved using:

sudo apt-get install sddm
sudo dpkg-reconfigure sddm