When you use the backend or frontend of your site (including the CRON script which uses the "legacy" mode, i.e. accessing your site's frontend) the hostname used by Akeeba Backup is what Joomla reports which is what PHP reports which is what your web server reports.
When you use the CLI CRON script we don't have access to the hostname. Instead, we use the hostname we stored into the database the last time you visited Akeeba Backup's Control Panel page.
You need to log into your site's administrator using the www subdomain. After logging in, do check check that you are still using the www subdomain.
Moreover, do check your configuration.php file. The $live_site line should be empty:
public $live_site = '';
If it has a URL, Joomla will use it instead of what your web server and PHP report as your hostname. If that has a URL like https://example.com (without www) it would explain your problem: Joomla reports the hostname example.com (this is used for backups run from the backend and frontend), but the hostname stored in the database (used by the CLI CRON job) would be the real hostname you are accessing the site on, i.e. www.example.com.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!