remote xdm kdm confusion

I have problems in accessing some SuSE V9.2 systems remotely. A X -query system :port works with all those X servers that were originally running SuSE 8.2 and were upgraded to SuSE 9.2; it fails to connect to all systems on which SuSE 9.2 was installed from scratch. What baffles me, is that if I choose xdm as the display manager on the server, it works, if I choose kdm as display manager, it doesn't. Where does the difference in behaviour come from? The display manager settings in YaST is on all systems exactly the same, /etc/sysconfig/diaplaymanager is identical, as is /etc/X11/xdm/xdm-config. I prefer kdm over xdm and would like to get it going. Thanks for any help Peter