VB.NET WinForms应用LDAP报错‘Server is not operational’求助

问题现象 #

• 一款运行多年正常的VB.NET WinForms应用，通过LDAP获取用户基础信息，在域控制器变更后出现异常
• 测试应用执行Clean Solution操作后，会触发"Server is not operational"报错，恢复清理掉的文件后可恢复正常
• 正式发布应用后，用户会在1-2天内收到该报错，之后又自动恢复正常

环境与排查信息 #

• 触发背景：新增部署两台域控制器（DC）并降级两台旧DC后，问题开始出现
• LDP.exe可正常连接LDAP服务
• Wireshark抓包结果：连接失败时仅发出"LDAP ping"，无后续通信；连接成功时则正常建立SASL/GSSAPI连接

测试应用代码片段 #

Dim ident As WindowsIdentity = WindowsIdentity.GetCurrent()
Dim username As String = ident.Name.Substring(ident.Name.IndexOf("\") + 1)
Dim protocol As String = "LDAP"
Dim path As String = "domain.com"
Dim sResult As SearchResult = Nothing
Dim ldapPath As String = If(String.IsNullOrEmpty(path), String.Format("{0}:", protocol), String.Format("{0}://{1}", protocol, path))
Using de As New DirectoryEntry(ldapPath)
    Try
        If de.Properties.Count > 0 Then
            Using adSearch As New DirectorySearcher(de)
                adSearch.Filter = String.Format("(&(objectClass=user)(sAMAccountName={0}))", username)
                sResult = adSearch.FindOne()
            End Using
        End If
    Catch ex As Exception
        Label1.Text = ex.Message
        
    End Try
End Using
Return sResult

已发现的相关情况 #

曾找到类似问题案例，仅通过重启操作解决，但未明确根本原因

请求 #

寻求该问题的根本原因及可行解决方案

内容的提问来源于stack exchange，提问作者SO User