Comment # 1 on bug 956068 from Dominique Leuenberger

As you installed KDE/Plasma first, I assume you're using sddm as your display
manager (the login manager)

Which means, this should be a duplicate of bug 931314;

but that bug is marked fixed with a corresponding fix released;

Can you please verify/confirm these things:
* are you using sddm as your display manager? (check
/etc/sysconfig/displaymanager)
* do you have the patch for gnome-session installed?

openSUSE-RU-2015:2018-1: An update that has one recommended fix can now be
installed.

Category: recommended (moderate)
Bug References: 931314
CVE References: 
Sources used:
openSUSE Leap 42.1 (src):    gnome-session-3.16.0-5.2

* if this is all correct, please poste the output of the systemd journal in
order to be able to further diagnose this issue.