Recursive method throws unspecified error on PropertyValueCollection assignment

Question

I am making an ASP.NET intranet website that uses Active Directory and I am trying to get all the groups a user belongs to, even sub groups etc ... So that means I had to make a recursive method to load all the groups in an ArrayList following the example given here : https://www.codeproject.com/Articles/18102/Howto-Almost-Everything-In-Active-Directory-via-C

Hence I have a method to recursively fill an ArrayList based on group membership:

public ArrayList AttributeValuesMultiString(string attributeName, string objectDn, 
                                            ArrayList valuesCollection, bool recursive)
{
    DirectoryEntry ldapConnection = new DirectoryEntry(objectDn);
    PropertyValueCollection valueCollection = ldapConnection.Properties[attributeName];
    IEnumerator en = valueCollection.GetEnumerator();

    while (en.MoveNext())
    {
        if (en.Current != null)
        {
            if (!valuesCollection.Contains(en.Current.ToString()))
            {
                valuesCollection.Add(en.Current.ToString());
                if (recursive)
                {
                    AttributeValuesMultiString(attributeName, "LDAP://" +
                    en.Current.ToString(), valuesCollection, true);
                }
            }
        }
    }
    ldapConnection.Close();
    ldapConnection.Dispose();
    return valuesCollection;
}

That I call from this other method:

public ArrayList Groups(string userDn, bool recursive)
{
    ArrayList groupMemberships = new ArrayList();
    return AttributeValuesMultiString("memberOf", userDn, groupMemberships, recursive);
}

Using the recursive boolean I can build my group's arraylist, but the first method can be used for other multistring object loading. Now when I test this just by a simple call to the method

//adManager is an instance of the class containing the methods above
//groups is an ArrayList
//testChain is my distinguishedName
groups = adManager.Groups(testChain, true);
foreach (var g in groups)
    Console.WriteLine(g.ToString());

I have the following exception:

System.Runtime.InteropServices.COMException : Unspecified error

And the point at which I have the exception is at the assignment:

PropertyValueCollection valueCollection = ldapConnection.Properties[attributeName];

I really don't see the problem, especially that this method was recommended on another SO thread, so I'm guessing it's functional

EDIT Seems that my problem comes from an authentication issue. I added impersonation in my test code but I still get an exception :

using (HostingEnvironment.Impersonate())
{
    var domainContext = new PrincipalContext(ContextType.Domain, "radiofrance.rootad.inetrf");
    var groupPrincipal = GroupPrincipal.FindByIdentity(domainContext, IdentityType.Name, "Users");
    if (groupPrincipal != null)
    {
        groups = adManager.Groups(testChain, true);
        foreach (var g in groups)
            Console.WriteLine(g.ToString());
    }
    else
        Console.WriteLine("Fail");
}