Can't query AD (get a DirectoryServicesCOMException)

I'm attempting to query AD in an ASP.Net (4.0) application that is running on Windows Server 2008 R2 (IIS7 installed). (It also fails when running as a 2.0 application as well)

This is nothing new for me, as I've done this many times before. I wrote a small ASP.Net program that runs fine on my own machine (Windows XP with IIS6), but fails when run on the 2008 box.

(The result is that you see a list of groups the user is a member of in a textbox)

(on button_click) 
var userName = txtUserName.Text;

if (userName.Trim().Length == 0)
{
     txtResults.Text = "-- MISSING USER NAME --";
     return;
}

var entry = new DirectoryEntry("LDAP://blah.blah/DC=blah,DC=blah",
                               "cn=acct, dc=blah, dc=blah",
                               "pass");

var search = new DirectorySearcher(entry);
search.Filter = "(SAMAccountName=" + userName + ")";
search.PropertiesToLoad.Add("memberOf");

var groupsList = new StringBuilder();

var result = search.FindOne();

if (result != null)
{
   int groupCount = result.Properties["memberOf"].Count;

   for (int counter = 0; counter < groupCount; counter++)
   {
           groupsList.Append((string)result.Properties["memberOf"][counter]);
           groupsList.Append("\r\n");
    }
}

txtResults.Text = groupsList.ToString();

When I run this code I get the following error on search.FindOne():

System.DirectoryServices.DirectoryServicesCOMException (0x8007203B): A local error has occurred.

   at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
   at System.DirectoryServices.DirectoryEntry.Bind()
   at System.DirectoryServices.DirectoryEntry.get_AdsObject()
   at System.DirectoryServices.DirectorySearcher.FindAll(Boolean findMoreThanOne)
   at System.DirectoryServices.DirectorySearcher.FindOne()
   at WebApplication1._Default.btnSearch_Click(Object sender, EventArgs e)

We've done a lot of research with this and twiddled every IIS7 setting we can think of, but no go so far. Any clues?

Change the username parameter from "cn=xxx, dc=yyy, dc=zzz" to "Domain\Username"