Ubuntu 15.04 release notes says
Known Issues
It is not all good news however. Here are the known issues. All of which affect every Ubuntu flavour.
**You may not be able to enter your pass phrase if you use full disk encryption.
LP: #1386005**
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.
Best Answer
There is a tutorial so you can fix it (beneath reinstalling). Just execute these two commands, it worked for me:
Link to the source/tutorial: http://dgzen.pw/2015/04/21/installing-spotify-in-ubuntu-15-04/