additional information:
I believe I have solved the problem (at least the cause): if I have 2 users as administrators (user1 and user2 for example), the course has been created by user1 and user2 tries to add lessons or sections, it is created but does not actually appear in the course.
Now, if I log in as user1 (who is the one who actually created the course), and add a new section/lesson, all the “disappeared” lessons and sessions reappear, even those created by user2.
I believe that with this information, you’ll be able to fix it.
Thanks