+++ This bug was initially created as a clone of Bug #1157431 +++

Using

 Alias=dbus-org.freedesktop.DisplayManager.service

in an own /etc/systemd/system/display-manager.service
and enabling this leads with

 qdbus --system org.freedesktop.DisplayManager
/org/freedesktop/DisplayManager/Seat0
org.freedesktop.DisplayManager.Seat.SwitchToGreeter

to

 Nov 21 11:54:55 tux sddm[4218]: Failed to read display number from pipe
 Nov 21 11:54:55 tux sddm[4218]: Display server failed to start. Exiting
 Nov 21 11:54:55 tux systemd[1]: display-manager.service: Main process exited,
code=dumped, status=6/ABRT
 Nov 21 11:54:55 tux systemd[1]: display-manager.service: Unit entered failed
state.
 Nov 21 11:54:55 tux systemd[1]: display-manager.service: Triggering OnFailure=
dependencies.
 Nov 21 11:54:55 tux systemd[1]: display-manager.service: Failed to enqueue
OnFailure= job: No such file or directory
 Nov 21 11:54:55 tux systemd[1]: display-manager.service: Failed with result
'core-dump'.

seems that sddm on leap 15.1 does call abort() (or the broken configured
libdbus-1 does this as this library is by default broken as a library sould
NEVER call at any point abort())