trust failure when trying to access my Kestrel-based REST server - rest

Question

I'm experimenting with a Xamarin app, which should access a .NET Core REST server.
I ran into this issue when switching to https; I can access the api from Chrome no problem, but if I try so from within my app, I get a System.Net.WebException saying
'Error: TrustFailure (A call to SSPI failed, see inner exception.)'.
I setup my server like this:
public static IWebHostBuilder CreateWebHostBuilder(string[] args)
{
var host = WebHost.CreateDefaultBuilder(args)
.UseUrls("https://*:5000")
.UseContentRoot(Directory.GetCurrentDirectory())
.UseStartup<Startup>();
return host as IWebHostBuilder;
}
and in my app, I do something like this:
public bool addUser(User user)
{
var request = WebRequest.Create("https://192.168.1.79:5000/api/users");
request.ContentType = "application/json";
request.Method = "POST";
try
{
var json = JsonConvert.SerializeObject(user);
var data = Encoding.UTF8.GetBytes(json);
using (var stream = request.GetRequestStream())
{
stream.Write(data, 0, data.Length);
}
using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
{
return response.StatusCode == HttpStatusCode.Created;
}
}
catch(Exception ecx)
{
var what = ecx.Message;
return false;
}
}
Thanks so much for any help!