Bug#801739: plasma-workspace: plasmashell crashes after login through display manager

[Stefan Rücker <s.ruecker@gmx.de>]

I have the same problem and opened a kde bug, although I don't know whether it 
is a kde or a debian problem:

https://bugs.kde.org/show_bug.cgi?id=353846

I've found this report after submitting the bug to bugs.kde.org.

Workaround conclusion:
When logging in into plasma with ligthdm or kdm add your user to the video 
group in /etc/group

When logging in into plasma with sddm add sddm to the video group in 
/etc/group

Cheers
Stefan