I Changed "Allow Free Registered Users (without subscriptions)" to "Yes" but this is not the answer. With that setting, even expired users are simply allowed to login and use the site as if they were not expired. I have 1 plan and it is set to renew at any time, no limitation to the number of occurrences of regular validity/price.
After testing, I have found that, if the subscription expires while the user happens to be logged in, then when they log out and login again, they are simply blocked. But if the subscription expires when they are NOT logged in, then everything works as it should. When they next login they are told that their subscription is expired and they are given the opportunity to renew.
After that happens, they can be renewed from the CB User Management in the backend, however upon every other expiration after that, whether they are logged in or not when it expires, they are simply blocked next time they login and not offered the chance to renew.