If you're able to try and step through the code, you can set breakpoints in the mojoPortal.Business.LdapHelper.cs file to trouble shoot the problem and see whats happening.
Possible in some AD configurations that the executing thread that is trying to query AD must be running in the context of a user with permission to query active directory. Typically by default on Win 2003 the thread is executing as IIS_WPG user, if you change it to a domain user its possible that will solve it.
Hope it helps,
Joe