using System;
using System.DirectoryServices;

namespace Tests.LDAP {
    class Program {
        static void Main(string[] args) {
            var server = "dc.domain.example.com";
            var port = 636;
            var container = "CN=Users,DC=domain,DC=example,DC=com";
            var path = $"LDAP://{server}:{port}/{{0}}{container}";
            var user = "Administrator@domain.example.com";
            var password = "s0m3p4ssw0rd";
            var de = new DirectoryEntry(string.Format(path, "CN=User Name,"), user, password, AuthenticationTypes.SecureSocketsLayer);
            var prop = de.Properties["cn"]; // <-- Hier wird die Exception geworfen
            Console.WriteLine($"cn: {prop}");
            Console.ReadKey();
        }
    }
}

Unhandled Exception: System.Runtime.InteropServices.COMException: 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.PropertyValueCollection.PopulateList()
   at System.DirectoryServices.PropertyValueCollection..ctor(DirectoryEntry entry, String propertyName)
   at System.DirectoryServices.PropertyCollection.get_Item(String propertyName)
   at Tests.LDAP.Program.Main(String[] args) in C:\source\Tests\Tests.LDAP\Program.cs:line 17