API REQUEST ERROR : "Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host."

I built a rest API that is used to send SMS messages and emails.I make an API call and get the following error:

"The underlying connection was closed: An unexpected error occurred on a receive."}
"Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host."

This is my code:

try{    var _client = new RestClient("https://xxxx:5000/test/api");    RestRequest request = new RestRequest("send-sms", Method.Post) { RequestFormat = DataFormat.Json };    request.AddJsonBody(new    {        MessageText = smsMessage.Body,        SenderName = smsMessage.SenderName,        RecipientPhone = smsMessage.To    });    var response = _client.Execute(request);    return response.IsSuccessful;}catch (Exception ex){    ExceptionHelper.OnlyWriteException(ex);    return false;}

It doesn't work only from this project, from curl and another empty project it does work.

What could be the reason it failed?