I am aving trouble getting ACS 4.2 to sync with domain controller running window 2008, I see the error listed below in the CS AUTH Logs when a user tries to authenticate via Windows external database:
CS AUTH Logs: AUTH 05/24/2010 11:08:19 E 2100 49316 0x2 External DB [NTAuthenDLL.dll]: Windows authentication FAILED (error 6L)
Win2008 is supported from ACS ver 4.2 patch 4 onwards, so you need to upgrade your ACS to 4.2(latest patch).
ACS 18.104.22.168 cumulative patch for windows can be downloaded from this link,
Also ACS is currently not supported on Win2008 R2 (ACS running on win2008 R2)
So make sure you are running acs 4.2.0 with latest patch or 4.2.1.
Do rate helpful posts