before line session optional pam_systemd.so otherwise - non-systemd user are faced with an error log line in syslog.
*** Bug 598346 has been marked as a duplicate of this bug. ***
Same goes for pam_elogind.so
I think kde@gentoo.org should also be Cc'd or perhaps even assigned to, as they are also listed as a maintainer and this bug has apparently gone unnoticed. Upstream agrees with the proposed fix: https://github.com/sddm/sddm/issues/881#issuecomment-329407384
This is fixed in sddm-0.16.0