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
