[Bug 1523070] [NEW] Plasma session does not start after upgrade to Kubuntu 15.10

[krusty]

Public bug reported:

I just upgraded to Kubuntu 15.10. There were no problems during the
upgrade itself. After the reboot I see the new login screen (sddm, used
to be lightdm before the upgrade).

I can enter my password but then nothing visibly changes. The screen
still shows the login screen, but nothing can be clicked. On some
occasions there is one change: the mouse cursor changes to an X.

I can go to Ctrl-Alt-F1 and login ok.

I have removed some old user configurations:
mv ~/.kde dot.kde.old
mv ~/.config dot.config.old
rm -rf ~/.cache
then rebooted and tried again, but the result is the same

I have switched back to lightdm:
dpkg-reconfigure lightdm
then
service sddm stop
service lightdm start
but the behaviour in lightdm is basically the same. I can enter the password then only the lightdm background is visible anymore and nothing furhter happens

What I see now is that after sddm or lightdm are in this post-password-nothing-happens-anymore state, I can go to Ctrl-Alt-F1 and do:
export DISPLAY=:0
xlogo
back to Ctrl-Alt-F7
and the logo shows ok.

In fact, it seems I can even do 
startkde
and the session comes up ok and now everything appers to be running fine. Hey I can even open a browser and report a bug!

** Affects: plasma-desktop (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1523070

Title:
  Plasma session does not start after upgrade to Kubuntu 15.10

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