Comment # 27 on bug 1156501 from Dr. Werner Fink

Hmmm ... I've added a line

  echo "Xsession: $DBUS_SESSION_BUS_ADDRESS" 1>&2

very first in /etc/X11/xdm/Xsession and a line

  echo "~/.xsession: $DBUS_SESSION_BUS_ADDRESS" 1>&2

very first in my ~/.xsession aka ~/.xinitrc and see

  Xsession: 
  ~/.xsession:
unix:abstract=/tmp/dbus-jG9lBsTQkx,guid=d755e5ba6e52c8e3e109d6155dce6f9b

in ~/.local/share/sddm/xorg-session.log ... that is that the sddm comes without
dbus connection and this triggers the script

  /etc/X11/xdm/scripts/20-dbus

I guess.