C# code cannot change AD password. (System.Runtime.InteropServices.COMException)

I'm trying to create a simple website to change my AD password. The website will be hosted internally in my IIS.

Here is how my UI looks like:

After clicking the submit button, I receive this error (which I displayed using an asp.net label):

System.Runtime.InteropServices.COMException (0x8007203A): The server is not operational.
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 changep1.changep.ChangeMyPassword(String domainName, String userName, String currentPassword, String newPassword) 

Here is my code:

using System.DirectoryServices;

protected void btncp_Click(object sender, EventArgs e)
{
    string result = "";
    string username = txtuser.Text;
    string oldpass = txtoldpass.Text;
    string newpass2 = txtpassnew2.Text;
    string dn = "Sales-comm";

    result =ChangeMyPassword(dn, username, oldpass , newpass2);
    lblresult.Text = result;

}


public string ChangeMyPassword(string domainName, string userName, string currentPassword, string newPassword)
{
    string resultinner="";

    try
    {
        string ldapPath = "LDAP://commonspace.Sales-comm.local/CN=admin,CN=Accounts and Services,DC=Sales-comm,DC=local";
        DirectoryEntry directionEntry = new DirectoryEntry(ldapPath, domainName + "\\" + userName, currentPassword);
        if (directionEntry != null)

        {
            DirectorySearcher search = new DirectorySearcher(directionEntry);
            search.Filter = "(SAMAccountName=" + userName + ")";
            SearchResult result = search.FindOne();
            if (result != null)
            {
                DirectoryEntry userEntry = result.GetDirectoryEntry();
                if (userEntry != null)
                {
                    userEntry.Invoke("ChangePassword", new object[] { currentPassword, newPassword });
                    userEntry.CommitChanges();
                }
            }
        }
    }
    catch (Exception ex)
    {
        resultinner= ex.ToString();
    }

    return resultinner;
}

I checked some Stack Overflow questions similar to mine, and changed the LDAP path accordingly, but I still can't solve this issue. Does anyone know what's wrong?

That means it can't contact the domain controller. So you'll have to do some diagnostics.

Do a DNS lookup and make sure you have results:

nslookup commonspace.Sales-comm.local

Try to connect to the various LDAP ports. I use the telnet client for this:

telnet commonspace.Sales-comm.local 389

These are the ports you can use:

LDAP: 389 Global Catalog: 3268 LDAP over SSL: 646 Global Catalog over SSL: 3269

If you find that 3268 is open, then you can bind using GC://:

string ldapPath = "GC://commonspace.Sales-comm.local/CN=admin,CN=Accounts and Services,DC=Sales-comm,DC=local";

If one of the SSL ports are open, then you need to use LDAP:// but include the port number:

string ldapPath = "LDAP://commonspace.Sales-comm.local:646/CN=admin,CN=Accounts and Services,DC=Sales-comm,DC=local";

If you go SSL, then may have to make sure that the root certificate that is used is trusted on the computer you are connecting from.