Ubuntu – Update from 14.10 to 15.04 result to a blank screen with cursor after sddm login

15.04kubuntunvidiaupgrade

I've made a long search thru all posts in the forum, and no one could help resolving my problem:

The machine is a desktop with only linux and an nvidia GeForce GT 430.

After the upgrade from kubuntu 14.10 to kubuntu 15.04, the new sddm login appear, then, after the login and the first loading bar, a blank black screen with only the active cursor appear, and only the alt+F2 works, even if the little window in the upper side doesn't execute any command.

The same upgrade in my notebook (with a dual linux-win and an nvidia Quadro 4000M) work just fine.

I've tried to purge and reinstall every version of the nvidia driver, I've tried to write the specifics of the monitor in the xorg.conf created by nvidia-xconfig, I've even purged and reinstalled the plasma-desktop, first the 5.2 then the 5.3 .

Nothing. No errors on dmesg, no errors on Xorg.0.log .

Now, can someone please suggest something that I can try? I would be really grateful. Thanks.

UPDATE:
as suggested by Zacharee1, I've installed the gdm and gnome-desktop packages and it works, so the problem is in the plasma-desktop. I've reinstalled sddm and plasma-desktop, but nothing has changed regarding sddm.
But now, if logged in in textual mode, with startx the graphic session works.

Best Answer

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.

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