Hi! Are you using Plymouth? If so, I might have resolved this issue already (not yet uploaded to the archive). Please try applying this patch to the /lib/systemd/system/sddm.service file: http://anonscm.debian.org/cgit/pkg-kde/kde-std/sddm.git/commit/?id=b50b826a4809b85270d5adc026b6d28fcdbb47c5 And see if that resolves the issue. Anyway, "failed to bring up the login screen" is very broad - what exacly happened? Did it crash? Did you get a tty, or just a black screen?
Cheers,
Matthias
