no way to protect a page: saving turns protection status to unprotected

usage manual of this plugin seems outdated. The admin interface of this plugin is very different.
So here's my problem: I created a standard paid membership and wanted to make a page available to members only.
In the "protected content" tab of the plugin admin menu, I changed the "modify access" for the page to protect.
When I edit the page, I try to enable protection. But when I publish the page, the protection switch turns back to grey (unprotected).
I don't understand. Really, I tried, but I don't get it.