diff -ur xdm.old/usr/lib/X11/display-manager xdm.new/usr/lib/X11/display-manager
--- xdm.old/usr/lib/X11/display-manager	2017-02-06 14:19:33.541035000 +0100
+++ xdm.new/usr/lib/X11/display-manager	2017-10-12 15:44:06.592204792 +0200
@@ -109,6 +109,8 @@
 
 #set -x
 
+# DISPLAYMANAGER used to be set in /etc/sysconfig/displaymanager, but the default is now set by update-alternatives
+DISPLAYMANAGER=$(realpath --relative-base=/usr/lib/X11/displaymanagers /usr/lib/X11/displaymanagers/default-displaymanager)
 DM=${DISPLAYMANAGER##*/}
 
 case "$DM" in
@@ -127,6 +129,7 @@
 	do
 	    name="${i}_vars"
 	    PIDFILE=/var/run/$i.pid;
+	    type $name > /dev/null || continue;
 	    $name $DM && break;
 	done
 	if [ -z "$DISPLAYMANAGER" -o ! -x "$DISPLAYMANAGER" ]
diff -ur xdm.old/var/adm/fillup-templates/sysconfig.displaymanager xdm.new/var/adm/fillup-templates/sysconfig.displaymanager
--- xdm.old/var/adm/fillup-templates/sysconfig.displaymanager	2017-01-04 11:15:56.259500000 +0100
+++ xdm.new/var/adm/fillup-templates/sysconfig.displaymanager	2017-10-12 16:35:23.775259305 +0200
@@ -1,14 +1,6 @@
 ## Path:	Desktop/Display manager
 ## Description:	settings to generate a proper displaymanager config
 
-## Type:	string(kdm,xdm,gdm,wdm,entrance,console,lightdm,sddm)
-## Default:	""
-#
-# Here you can set the default Display manager (kdm/xdm/gdm/wdm/entrance/console).
-# all changes in this file require a restart of the displaymanager
-#
-DISPLAYMANAGER=""
-
 ## Type:	yesno
 ## Default:	no
 #