We are seeing this error where you try and enter the class to moderate from admin and you get a 404 error page.
This is only an issue in admin, the instructor and students still get to join so here are the steps to take.
1. Get the free 100% code from the instructors class (login to that class edit page via the admin and go to the bottom)
2. Log out of their account and go to the class in the class database.
3. Register to the class with the free code with your moderator account.
You will have moderator privileges if you make sure to login with your instructor account - don't do it with a student account.