IIS-Windows-Authentication stuck in an infinite loop

I'm trying to access an old ASP.NET-Api(API 1) via Angular 8. Because of CORS-Issues the access is handled via a proxy.conf.js-File. (Found in another post)

var Agent = require("agentkeepalive");

var keepaliveAgent = new Agent({
  maxSockets: 100,
  keepAlive: true,
  maxFreeSockets: 10,
  keepAliveMsecs: 1000,
  timeout: 60000,
  keepAliveTimeout: 30000 // free socket keepalive for 30 seconds
});

var onProxyRes = function (proxyRes, req, res) {
  var key = 'www-authenticate';
  proxyRes.headers[key] = proxyRes.headers[key] && proxyRes.headers[key].split(',');
};

const PROXY_CONFIG = [
  {
    target: Application-Url,
    context: "/api/",
    secure: false,
    changeOrigin: true,
    auth: "LOGIN:PASS",
    loglevel: "debug",
    onProxyRes: onProxyRes,
    agent: keepaliveAgent
  }
];
module.exports = PROXY_CONFIG;

In the .NET-Application the only thing that hints to the authentification is the following line in the web.config-File.

<authentication mode="Windows" />

When compiled and executed the following behavior occurs: A login-mask shows up which asks for windows-authentication. If the credentials are entered the mask closes and reopens again instantly. This behaviour continous endlessly.

Note: Accessing the api-endpoints via the commandline of the browser works perfectly fine.