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

Active directory search won't run on client computer

Active Directory search works fine on dev machine with debugger, and on the web server. It breaks when you try to use the page from another machine.

The forum post says to check "Trust computer for delegation". I don't know what delegation is, so this likely wouldn't fix anything I'm working on, but it's something to try if nothing else works, I suppose.

http://forums.asp.net/t/1062417.aspx

1. I went into Active Directory and opened up the computer account for the web server. I checked the box which says "Trust computer for delegation."
2. I opened up my user account and checked the box "Account is trusted for delegation."

Search works from a windows application, but not through ASP.NET

If you can connect to Active Directory using a windows app, such as the Softerra browser, but not through ASP.Net, it is often a permissions issue (according to my reading, anyway. I have no actual experiance). It often happens when you are connecting to LDAP annonymously.

To fix this, create a system account that only has read access to active directory. Always pass this username and password when connecting. If you can't do this, these two pages have helpful hints.

LDAP connection FAQ, Forums.ASP.Net, Microsoft's DirectoryServices connection help page. (Most of the problems on these pages can be avoided by connecting with a username and password)