I have a client on SAP version 9.1 PL04.
Two of their machines this morning when logging on first thing had error message "Failed to Connect to System landscape Directory: Please contact your administrator"
Other machines unaffected.
We restarted SLD and Licence Manager without any change.
Remembering the Security Certificate problem a while ago we looked to see if there were any recent updates and there were. The machines that were working did not have these updates.
We removed these updates and the user could login to SAP again.