Ajax url moved via 301 and request throws error

Question

I use this script, to get some data via ajax request:

jQuery(document).ready(function() {
    $urlpre = document.location.protocol;
    $.ajax({
        url: $urlpre+'//domain.tld/file.php',
        error: function (thrownError) {
            pushNotify('Could not connect to update server!', 'danger', '5');
        },
        success: function (data) {
            ...
        }
    });
});

Now the url changed from domain.tld/file.php to sub.domain.tld/file.php. I set up a 301 redirect and in the browser this works fine, but the ajax request throws me the error. I tried to google the issue, some have problems with http: and https:, but I don't think that's my case.