Hi Boudewijn,
I just tried this on my test site and it’s working fine.
Could you try the following and then check if the text shows up when you view the site while you’re logged in?
[s2If !is_user_logged_in()]
Content for anyone that is logged in, regardless of their Membership Level.
[/s2If]
If that doesn’t work either, then I suspect you have something else going on. There could be another plugin installed that’s causing a conflict with the parsing. You might want to use something like the Plugin Organizer plugin to disable all plugins on a specific page and then try viewing that page and seeing if the widget works. If it does, then you should try re-enabling each of the plugins until you find the conflicting one.
Also, Eduan was correct: In the PHP example you gave, you should be using == (comparison operator) not = (assignment operator).