Hi Valérie. Thanks for the detailed information.
The Events Calendar core plugin is required for the PRO extension to work, but not vice-versa; PRO is an add-on to the core plugin.
Adding events will work without PRO activated.
However, I want to help you get the PRO add-on working, of course!
Your screenshot showed things working correctly, but the link to http://libredetre.com/agenda/ didn’t have any content on it. I did find http://libredetre.com/events/ from your website’s navigation menu.
That events link seems to be working fine.
Is the issue still happening on your site? If yes, is it when the Photo view is enabled? Please explain how to re-create the issue you’re experiencing.
Thanks.