GUI won't start after upgrade to Kubuntu 16.10 Beta 2

[Hans-Peter]

Today I tried to upgrade to Kubuntu 16.10 Beta 2. I did it by using do-release-upgrade.
Upgrade went fine, no issues whatsoever. But now GUI won't start. When I boot the box, the glowing "Kubuntu" appears. When it disappears, there is a cursor on the top left of the screen.
I get a console with CTRL-ALT-F1 but can't start X. Xorg.0.log doesn't show any errors.

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. It seems that your bug report is not filed about a specific source package though, rather it is just filed against Ubuntu in general. It is important that bug reports be filed about source packages so that people interested in the package can find the bugs about it. You can find some hints about determining what package your bug might be about at https://wiki.ubuntu.com/Bugs/FindRightPackage. You might also ask for help in the #ubuntu-bugs irc channel on Freenode.

To change the source package that this bug is filed about visit https://bugs.launchpad.net/ubuntu/+bug/1629788/+editstatus and add the package name in the text box next to the word Package.

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[Hans-Peter (hanspeterg)]

When starting lightdm from console I am able to login to the GUI. So no greeter is called from the boot / init process. I have a /sbin/init/splash (with pid 1) running all the time. Maybe this is the reason why it hangs?

[Hans-Peter (hanspeterg)]

After a reboot I got a message instead of the cursor: Something (I can't remember exactly what is was) failed and I should issue " systemctl status sddm.service " to examine the situation.
The output showed, that sddm.service was requesting the contents of the /etc/X11/default-display-manager file. In my case the file contained 'lightdm'. I changed this to sddm and now everything is fine.