The solution exposed in this forum, works for me.

I just had to set the parameter 
    proxy_mode = True  
in the odoo.conf

The
     proxy_set_header X-Forwarded-Host $ host 
was already in my Nginx configuration

\https://stackoverflow.com/questions/51675245/mixed-content-warning-for-insecure-resources-from-odoo

I have the same problem on my audio 13 communient y a priori I am behind a web proxy haproxy so it may be an infrastructure problem .I will take a closer look but nothing is displayed on my side from the beginning .

Hi Bruce:

The eLearning app seems to be a bit finicky. Check whether the Course and all the Content Items within the Course are published. I know you have mentioned that you have, but sometimes the app does not seem to be registering it properly.

You can quickly check this by doing either of the following.

Go to eLearning, edit the Course in question and check whether the count in the contents Smart Button to the top right of the Course Title displays the correct count i.e. the number of content items in the Course. If the count does not match the number of Content Items in the Course, it is an indication that some of the Content Items did not get published. Go ahead and publish them individually.

OR

Go to the Website, click on Courses, select the Course in question and check whether the Course Items have an unpublished status. If they do, go ahead and publish them individually.