Slim REST API调用错误，没有错误消息（JQuery）

I tried to make a GET call from a self written API in javascript (JQuery). I get an error but there is no message included.

Client code looks like this

$(document).ready(function () {
        $.ajax({
            type: "GET", 
            url: "APIURL...",
            headers: {"Content-Type": "application/json;odata=verbose", "Accept": "application/json;odata=verbose", "crossDomain": "true", "credentials":"include"},
            success: function (data) {
                console.log(data);
            },
            error: function (jqXHR, textStatus, errorThrown) {
                console.log(textStatus); //output: error
                console.log(errorThrown); //output: 
            }
        });
    });

Also tested this with another url of a REST Test API https://jsonplaceholder.typicode.com/todos/1. This works fine, it outputs the expected data in the success function. So I think the problem is somewhere in my own API.

I made the API with PHP and the Slim Framework. Code looks like this

$app->get ( '/test', function ($request, $response) {
    $data["test"] = "data";
    $json = json_encode ( $data );
    $response->write ( $json );
    return $response;
});

This is the called function from my jquery code. In the browser its output is as expected {"test":"data"}

The server where my API runs is reachable from the internet. Any solutions or suggestions?