JOOMLA End of script output before headers: index.php

Question

I have a joomla website running in MAMP, but I can't get it to work.
I checked the APACHE log and found this error:

[core:error] [pid 17696:tid 1216] [client ::1:52925] End of script output before headers: index.php

This is the error I get when opening my site:

Im using joomla ‎4.2.9
PHP version 8.0.1

The contents of my index.php:

<?php

/**
 * @package    Joomla.Site
 *
 * @copyright  (C) 2005 Open Source Matters, Inc. <https://www.joomla.org>
 * @license    GNU General Public License version 2 or later; see LICENSE.txt
 */

// NOTE: This file should remain compatible with PHP 5.2 to allow us to run our PHP minimum check and show a friendly error message

// Define the application's minimum supported PHP version as a constant so it can be referenced within the application.
define('JOOMLA_MINIMUM_PHP', '7.2.5');

if (version_compare(PHP_VERSION, JOOMLA_MINIMUM_PHP, '<')) {
    die(
        str_replace(
            '{{phpversion}}',
            JOOMLA_MINIMUM_PHP,
            file_get_contents(dirname(__FILE__) . '/templates/system/incompatible.html')
        )
    );
}

/**
 * Constant that is checked in included files to prevent direct access.
 * define() is used rather than "const" to not error for PHP 5.2 and lower
 */
define('_JEXEC', 1);

// Run the application - All executable code should be triggered through this file
require_once dirname(__FILE__) . '/includes/app.php';

I searched the past 4 hours on the internet for solutions, but nothing worked.
Is there anything I could do to fix this?