Event ID 9646 started to appear on my SBS 2003 server recently. XP and Win 7 clients were affected. I disabled all offloading settings on the server and workstation NIC's. Thought this should fix it. This morning came in to see another user locked out of Outlook with 9646 in the event log.
So... restarted the Microsoft Firewall service. FIXED... All of the 32+ logons in Exchange system manager were gone. Only one active logon remained.