Check the cache

Apparently, LDAP can sometimes get messed up when windows caches the LDAP information. Somehow. This really doesn't make any sense.

1. Go to the registry : HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ADs\Providers\LDAP\distinguished name of the schema container
2. Here you'll find a string called File which contains the location of the schema. Usually it says : %SystemRoot%\SchCache\[server_name].sch
3. Rename or delete this file.

http://forums.asp.net/t/1011119.aspx