This is just an integration problem: Fun MOOC hosts the main navigation and manage user subscriptions. Then Inria hosts both the jupyterhub instance and the discourse forum on its own server and we need a transparent way to propagate the user’s authentication information to those platforms when they show as embedded element in the Fun MOOC pages (using iframes).
Safary became more stringent and no longer allows third party cookies which is the way this transparent integration between the different systems work. The other browsers (independently of the operating system) all allow third party cookies for the time being (unless you disable them in the configuration or via privacy. preserving extensions).
We will probably need to find another way to do this integration in the future, but for the time being, just install firefox for this mooc session (or use any other browser of you choice except safary).