"I'm not sure if that bug is in 0.5.1. It was in 0.6.0. What worked for me was to add user sddm to video group in /etc/group"
Hmmm...I have this entry in /etc/group sddm:x:131: is this what you are talking about?
Otherwise I could do adduser sddm video as root.
Also, without changing anything in my sources.list, I could download more updated packages from unstable and install with dpkg.
What do you think?
Thank you.