Many thanks Didi, I can confirm that workaround "adduser sddm video" fixed the problem.
- Bug#801106: sddm-greeter fails to start Andy Wood
- Bug#801106: sddm-greeter fails to start Diederik de Haas
- Bug#801106: marked as done (sddm-greeter f... Debian Bug Tracking System
- Bug#801106: sddm-greeter fails to start Andy G Wood