jQuery .ajax POST返回null

Question

I am trying to post some data to my server and get some data back. For some reason, the data I am posting comes back as null:

The jQuery:

var data = { message1: "bla", message2: "blabla" };

$.ajax({
    type: 'POST',
    url: 'https://example.com/api',
    data: JSON.stringify(data), // EDIT: replacing this with [data: data,] also does not help (also not in combination with omitting the following line)
    contentType: 'application/json', // EDIT: I have removed this line now, but the problem persists
    dataType: 'json',
    success: function(response) {
      console.log(response);
    }
  });

The PHP:

$response['message1'] = $_POST['message1'];
$response['message2'] = $_POST['message2'];
$response['message3'] = 'blablabla';

print_r(json_encode($response));

This returns:

{ message: null, message2: null, message3: "blablabla" }

And I want:

{ message: "bla", message2: "blabla", message3: "blablabla" }

I have already tried all sorts of combinations of quotes in my data (e.g. '{"message1": "bla" (...)}' and omitting the JSON.stringify), but nothing seems to work..