A Typo in the source file src/daemon/LogindDBusTypes.cpp prevents detection of ConsoleKit thus Seat0 is never created nor is the DBUS service to start new session created.
Pleas attach the included patch to the ebuild while we are waiting for the SDDM team to relase a fix.
The patch affects a line that is mentioned but not changed in the existing patch sddm-0.16.0-ck2-revert.patch, I would guess that the existing patch shall be applied first.
The upstream issue is: https://github.com/sddm/sddm/issues/1027