I need to install the demo content manually.

There is an error with the plugin "Iron Demo Importer".

I got this error: "Important: HTTP Loopback Connections are not enabled on this server. If you need to contact your web host, tell them that when PHP tries to connect back to the site at the URL http://podbox.dk/wp-admin/admin-ajax.php and it gets the error Response code: 500. There may be a problem with the server configuration (eg local DNS problems, mod_security, etc) preventing connections from working properly."

I tried to downgrade my PHP version to 7.4 to check if it will resolve the issue.
It did! 

But but during the installation of demo content again, it reached a timeout after 29 seconds with the error: "Maximum execution time of 29 seconds exceeded in /customers/1/c/9/podbox.dk/httpd.www/wp-includes/class-wp-image-editor-imagick.php on line 386".

The max_execution_time for my PHP server is set to 300 and not possible to change.

What do I do now?

It's working now...!!!

on functions.php on theme add this

add_filter( 'wp_image_editors', 'change_graphic_lib' );
function change_graphic_lib($array) {
return array( 'WP_Image_Editor_GD', 'WP_Image_Editor_Imagick' );
}

or add this line on .htaccess

SetEnv MAGICK_THREAD_LIMIT 1