We have a .NET 6 application which makes requests to a third party API.
About a month ago, requests from most of our computers started failing. The error we get is “System.Net.Http.HttpRequestException: An error occurred while sending the request. —> System.IO.IOException: The response ended prematurely.”

That’s it, that’s the whole error. No inner exception, unless I don’t know how to log it properly.

Only thing that has changed is the third party API started using Cloudflare.

Things that have been tested and didn’t help:

• Update Windows version to 22H2

• Install all Windows updates

• Check firewall

• Compare registry with IIS Crypto

• Reinstall our application

• Compare .NET runtimes and make them equal

• Try to launch our app as self-contained and see if that changes anything

• Check if our external IP is blocked by the third party

• Check TLS (requests failed with 1.1, 1.2, 1.3)

• Check computer firewall

• Windows proxy – it’s off

• Network adapter settings

• ipconfig /flushdns

• Reset windows proxy

What I also tried is to make a .NET console application and just send a request straight to the third party.
Code:

try
{
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
    ServicePointManager.Expect100Continue = true;
    ServicePointManager.DefaultConnectionLimit = 9999;

    var apiUrl = "https://sales.gjensidige.lt/claimsapi/";

    var httpClient = new ServiceCollection()
            .AddHttpClient()
            .BuildServiceProvider()
            .GetService<IHttpClientFactory>()
            .CreateClient();

    httpClient.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("text/xml"));
    httpClient.Timeout = new TimeSpan(0, 0, 30);

    var httpContent = new StringContent(body, Encoding.UTF8, "text/xml");

    var httpRequest = new HttpRequestMessage
    {
        //Version = HttpVersion.Version20,
        RequestUri = new Uri($"{apiUrl}PharmacyClaim/RegisterClaim"),
        Content = httpContent,
        Method = HttpMethod.Post
    };

    Log.Logger.Information("Making request to {0}, content:{1}{2}", httpClient.BaseAddress + "PharmacyClaim/RegisterClaim", Environment.NewLine, body);

    var response = await httpClient.SendAsync(httpRequest);
    var content = await response.Content.ReadAsStringAsync();

    Log.Logger.Information("Response content: {0}{1}", Environment.NewLine, content);
    Log.Logger.Information("Response info: {0} HTTP version: {1}", Environment.NewLine, response.Version);
}
catch (HttpRequestException e)
{
    Log.Logger.Error(e.InnerException, "Caught inner exception: {0}", e.InnerException?.Message);

    throw;
}
catch (Exception ex)
{
    Log.Logger.Error(ex, "Caught exception: {0}", ex.Message);

    throw;
}

Body is just an XML string.

I’m getting the same error, but what I could figure out is:

• setting HTTP protocol version to 2.0 fixes the problem, but in some computers the request works with HTTP 1.1, so this really is just fixing the cause, not the reason

• launching fiddler in PC makes the request work – I’ve read a bit about this and it seems like fiddler makes some kind of proxy for requests? No idea why using it makes the request work, but it seems like Fiddler alleviates a lot of .NET HTTP problems

The strangest thing is that in some computers the requests work, but in most of them they started failing.

Is this a Cloudflare issue, a .NET one, network issue? Can I get a more precise exception?

Any and all help would be appreciated.