That's usually caused by redirect configured somewhere or CBSubs isn't publicly accessible. If the URL has an Itemid check what menu item it's associated with and be sure that menu item is public.

Ensure the URL can be accessed by a non-logged in user. Ideally open it in your browser as incognito. If it works then it's accessible. If it doesn't then you've something blocking its access, which is usually the menu item it's associated with (check the Itemid in the URL). As for CRON use a wget call. See the below topic for examples.

www.joomlapolis.com/forum/277-cb-paid-subscriptions-support/237481-cron-job#297853

I don't know what to tell you. If it runs in the browser then it's working fine, but that will come with whatever permissions you have at the time of being logged in. It's just an empty URL that triggers behavior in CBSubs. wget should be able to call it fine. We've been using it for over 10 years here at Joomlapolis fine.

My guess is you've something configure or installed denying access. CBSubs itself doesn't have a single 403 header anywhere in it. 403 is usually caused by Joomla itself denying access. Do you have any sort of security extension installed as maybe that's doing it?

Try adding an &Itemid=MENU_ID to the URL with a menu id that's publicly accessible like your home page maybe. It's possible you also have the CRON configured incorrectly for the URL.