XMLHttpRequest cannot load wp-admin/admin-ajax.php

Embedding tweets and youtube videos, as well as adding new themes, stopped working (when I press on “Add new” I get “An unexpected error occurred. Something may be wrong with www.ads-software.com or this server’s configuration.”).

I followed the loading of my site (https://soltys.cs.csuci.edu) on Safari and Chrome, and using the development window I found the following error: “XMLHttpRequest cannot load …/wp-admin/admin-ajax.php due to access control checks”.

I really don’t know how to fix it (I tried this: https://wordpress.stackexchange.com/questions/215053/cannat-load-admin-ajax-php-no-access-control-allow-origin , but it didn’t help).

I am running:
Ubuntu Linux 16.04
Server: Apache/2.4.18
PHP: v7.0.15-0ubuntu0.16.04.4