Hello.
Two people have just commented in the spanish forums (https://moodle.org/course/view.php?id=11) the same problem: students can see the link "Add an activity or resource" in every section of their courses, after Moodle has been upgraded to version 3.11+.
Then, I have upgraded mi localhost Moodle to test the issue and I've confirmed this problem. I think it must be a bug of the version.
When students clics on that link they can see the selection window, but empty.
Thanks !