No luck in adding a issue via AJAX and the REST API. I can get it to work with Postmen, unfortunatly, can't get it with an Ajax request.
The JSON I create is fine, the post request also. The issuetype is something I created myself, using Bug gives the same problem. See the JSON object created, my error and my code: JSON object (this is a snippet from console.log):

The Error
0: "Unrecognized token 'fils5poet5': was expecting 'null', 'true', 'false' or NaN↵ at [Source: org.apache.catalina.connector.CoyoteInputStream@7b958ed2; line: 1, column: 21]"
jira = {
   fields : {
      project : {
         key : "CIC"
      },
      summary : "test",
      description: "test",
      issuetype : {
         name : "Sandbox item"
      }
   }
};
console.log(jira); //Also see image at top of this post.
// Submit to Jira api
$.ajax({
   type : "POST",
   dataType : "JSON",
   url : configuration.api_url,
   beforeSend:  function (xhr) {
      xhr.setRequestHeader ("Authorization", "Basic ItsAWrap!(itworks...)"),
      xhr.setRequestHeader ("Content-Type", "application/json");
   },
   data : jira,
   success : (function(response) {
//Do something
}})
 
     
    