here's an explanation of the issue
https://stackoverflow.com/questions/6186770/ajax-request-returns-200-ok-but-an-error-event-is-fired-instead-of-success

IMO https://github.com/redmine/redmine/blob/dcb545efadc586ab15065870b943763263ed9a46/app/controllers/application_controller.rb#L659 any maybe other places should be changed to return https://httpstatuses.com/204 (no-content) or a valid json {} if the dataType is json

example

    $.ajax({
        type: "DELETE",
        url: '/issues/5.json'
        dataType: 'json'
        success: function (data) {
            alert("ok");
        },
        error: function (textStatus, errorThrown) {
            alert('error');
        }
    });

if fails as an error, because the returned body is blank (not a valid json), but the request was successful

this behaviour is documented
http://api.jquery.com/jquery.ajax/

dataType (default: Intelligent Guess (xml, json, script, or html))
Type: String
The type of data that you're expecting back from the server. If none is specified, jQuery will try to infer it based on the MIME type of the response (an XML MIME type will yield XML, in 1.4 JSON will yield a JavaScript object, in 1.4 script will execute the script, and anything else will be returned as a string).

"json": Evaluates the response as JSON and returns a JavaScript object. Cross-domain "json" requests that have a callback placeholder, e.g. ?callback=?, are performed using JSONP unless the request includes jsonp: false in its request options. The JSON data is parsed in a strict manner; any malformed JSON is rejected and a parse error is thrown. As of jQuery 1.9, an empty response is also rejected; the server should return a response of null or {} instead. (See json.org for more information on proper JSON formatting.)

I propose to change

def render_api_ok
  render_api_head :ok
end

to

def render_api_ok
  render_api_head :no_content
end