System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Authentication failed because the remote party has closed the transport stream.
   at System.Net.TlsStream.EndWrite(IAsyncResult asyncResult)
   at System.Net.ConnectStream.WriteHeadersCallback(IAsyncResult ar)
   --- End of inner exception stack trace ---
   at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)
   at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)
   --- End of inner exception stack trace ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Insurance.Controller.ProposalController.<UploadfileondropboxnewAsync>d__6.MoveNext() in C:\CODES\MailProject\Insurance\Controller\ProposalController.cs:line 205