From what I have found you can configure mime types in IIS, if that is possible for you:

web.config

<configuration>
  <system.webServer>
    <staticContent>
      <mimeMap fileExtension=".css" mimeType="text/css" />
      <mimeMap fileExtension=".js" mimeType="text/javascript" />
    </staticContent>
  </system.webServer>
</configuration> 

You may also want to add the charset (apply the correct one for you if it's not utf-8):

<configuration>
  <system.webServer>
    <staticContent>
      <mimeMap fileExtension=".css" mimeType="text/css; charset=utf-8" />
      <mimeMap fileExtension=".js" mimeType="text/javascript; charset=utf-8" />
    </staticContent>
  </system.webServer>
</configuration> 

If it's not that maybe the response is really an HTML page with an error text?
It might be an HTML page in disguise.

Credits:

• web.config example on SO
• mimemap docs
• iis configuration files

You probably did not add the appropriate mime type on the server side and the browser is sending it with "text/css".

You can always get the content in plain text and put it in a style tag.

var xhtp = new XMLHttpRequest();
xhtp.open("GET","/Content/css-app/bootstrap.min.css?v=20210924122520");
xhtp.setRequestHeader("Content-Type","text/plain");
xhtp.onload = function(){
    let stylTag = document.createElement("style");
    stylTag.textContent = this.responseText;
    document.head.appendChild(stylTag);
};
xhtp.send();