Estou lutando por horas agora. Quero fazer uma solicitação simples de ajax para outro domínio, mas recebo o erro http 401 o tempo todo:

jQuery(document).ready(function($){
  var challengeid = $('#codepressHook').data('challengeid');
  var clicked = false;
  $('#codepressHook').click(function(){
    if(!clicked){
      $.ajax({
        url: "https://dev.radbonus.com/admin/affiliate-connections/retrieveSingle/"+challengeid+".json",
        method: "GET",
        dataType: "json",
        jsonp: false,
        contentType: "application/json",
        xhrFields: {
          withCredentials: true
        },
        beforeSend: function(xhr){
          xhr.setRequestHeader("Authorization", "Basic "+ btoa(username+":"+password));
        },
        success: function(data){
          $('#codepressHock').html(data.data.code);
        },
        error: function(error){
          alert(error);
        }
      });
    }
  });
});

Defino todos os cabeçalhos CORS relevantes no lado do servidor. Aqui está o tráfego de rede:

Request URL:https://dev.radbonus.com/admin/affiliate-connections/retrieveSingle/45.json
Request Method:OPTIONS
Status Code:401 Unauthorized
Remote Address:185.102.94.230:443
Referrer Policy:no-referrer-when-downgrade

Response Headers
view source
Access-Control-Allow-Credentials:true
Access-Control-Allow-Headers:Content-Type, X-Requested-With, Authorization, Origin
Access-Control-Allow-Methods:POST, GET, PUT, DELETE, OPTIONS
Access-Control-Allow-Origin:http://radbonus.com
Access-Control-Max-Age:31536000
Content-Length:463
Content-Type:text/html; charset=iso-8859-1
Date:Sat, 24 Jun 2017 11:25:33 GMT
Server:Apache/2.4.18 (Ubuntu)
WWW-Authenticate:Basic realm="Admin"

Request Headers
view source
Accept:*/*
Accept-Encoding:gzip, deflate, sdch, br
Accept-Language:de-DE,de;q=0.8,en-US;q=0.6,en;q=0.4
Access-Control-Request-Headers:authorization,content-type
Access-Control-Request-Method:GET
Connection:keep-alive
Host:dev.radbonus.com
Origin:http://radbonus.com
Referer:http://radbonus.com/plugintest/
User-Agent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36

Eu sei que existem muitas postagens sobre esse tópico, mas parece que estou perdendo algo simples. Alguém pode ajudar-me?