HttpClient and WebClient requests / responses don't work for intranet with DefaultCredentials - .net-core

Question

Problem: I am unable to get content from an company internal webpage through using HttpClient or WebClient. I am able to get the content by accessing the URL directly, however.
Details: .NET Core 3.1 Razor Pages, IIS 10, Windows Authentication.
I have a website http://myintranet/Editor/Bib/4343 where a user can press a button to generate a static page. Behind the scenes, it attempts to read a stream from http://myintranet/Editor/NewBib/4343/true and create a static HTML page from it.
When clicking the button, the response is always IIS 10.0 Detailed Error - 401.1 - Unauthorized etc.
However when I access the webpage directly in the browser, it opens up just fine (note that if it is the first time accessing the website, I am prompted by the browser to enter my username and password. After that, the browser remembers these credentials).
Also note that when running it from localhost through Visual Studio, all works fine, the static page downloads properly too.
Here is my code:
Version 1:
public IActionResult OnPostGenerateStaticPage()
{
try
{
HttpClient client = new HttpClient(new HttpClientHandler() { UseDefaultCredentials = true, PreAuthenticate = true });
HttpResponseMessage response = client.GetAsync(Url.PageLink().Replace("Bib", "NewBib") + "/true").Result;
var indexPageContents = response.Content.ReadAsStreamAsync().Result;
var cd = new System.Net.Mime.ContentDisposition
{
FileName = BibNumber + ".htm",
Inline = false,
};
Response.Headers.Add("Content-Disposition", cd.ToString());
return File(indexPageContents, "text/html");
}
catch (IOException)
{
return RedirectToPage("./Bib");
}
}
Version 2:
public IActionResult OnPostGenerateStaticPage()
{
try
{
WebClient client = new WebClient { UseDefaultCredentials = true };
string desiredUrl = Url.PageLink().Replace("Bib", "NewBib") + "/true";
var indexPageContents = client.OpenRead("desiredUrl");
var cd = new System.Net.Mime.ContentDisposition
{
FileName = BibNumber + ".htm",
Inline = false,
};
Response.Headers.Add("Content-Disposition", cd.ToString());
return File(indexPageContents, "text/html");
}
catch (IOException)
{
return RedirectToPage("./Bib");
}
}
Another thing, I have asked the web server admin to check that NTLM is above Negotiate for the authentication providers for this website and it is. Also, Anonymous and Basic Authentication are disabled and Windows Authentication is enabled.
Not sure where to go from here...