I can retrieve all user names from active directory group on local server but on IIS showing exceptional error

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IIS

LDAP

web.config

I am new user for stachoverflow, I am developing one web application for intranet in which it authenticate user from active directory which works fine on IIS but I am getting problem to retrieve data(all user names) from group of active directory which works fine on local server. on IIS it gives exceptiona error- System.DirectoryServices.DirectoryServicesCOMException: Logon failure: unknown user name or bad password. in IIS windows authentication enabled and other are disabled.

My code is:
Web.config file:

<authentication mode="Windows">
            <forms loginUrl="~/TTracker/Login.aspx" timeout="600"></forms>
        </authentication>

<authorization>
            <deny users="?"/>
            <allow users="*" />


        </authorization>
    <identity impersonate="true" />

Code for retrieving data:

using System.DirectoryServices;
using System.DirectoryServices.AccountManagement;
using (HostingEnvironment.Impersonate())
            {
                DropDownList1.Items.Add(new ListItem("-Select-", ""));
                string grpname = "Group1";
                PrincipalContext ctx = new PrincipalContext(ContextType.Domain, "domain");
                GroupPrincipal grp = GroupPrincipal.FindByIdentity(ctx, IdentityType.SamAccountName, grpname);
                if (grp != null)
                {
                    foreach (Principal p in grp.GetMembers(false))
                    {
                        DropDownList1.Items.Add(p.SamAccountName + "-" + p.DisplayName);

                    }
                    grp.Dispose();
                    ctx.Dispose();
                    Console.ReadLine();
                }
                else
                {
                    Console.WriteLine("We did not find that group.");
                    Console.ReadLine();
                }
            }

this is my code for retrieving data from active directory which works fine locally but not works on IIS.

It will be great help if any one solve this issue.

Thanks in advance.