Currently the default HTTP version for HttpClient is 1.1

• https://source.dot.net/#System.Net.Http/System/Net/Http/HttpClient.cs,e109da57e2ee7ede

In both examples you are using version 1.1.

The resource you are trying to fetch has response version of HTTP 2.0, try sending the request with

var request = new HttpRequestMessage(HttpMethod.Get, "http://131.189.89.86:11920/SetupWizard.aspx/yNMgLvRtUj")
{
    Version = new Version(2, 0) // change the version
};
HttpResponseMessage response = await client.SendAsync(request);

Here is the place where the exception occurs:

• https://source.dot.net/#System.Net.Http/System/Net/Http/SocketsHttpHandler/HttpConnection.cs,5512bd8ec864de48,references