Thank you for the quick reply!
There are three websites, all setup in the same manner (hosting, content, plugins, including yours).
One of them fails to hide content. It only does it for the first visit, after I click the save button inside the plugin. That is: I click Save and then it’s protected for one first visit, per user.
One of the blocked countries is the one I live in. The hidden area is the front end.
I test using another browser where I’m not logged in. And it works fine, for the other two.
I also ask my client to test, he’s on another ISP from the same country and registers the same behavior.