Hey Arnaud and sorry for the trouble here!
I see exactly what you mean: the titles of your events are not displayed.
It appears that your theme has some custom styles that are overriding and conflicting with the calendar’s styles.
To test that, will you please switch to the default WordPress Twenty Sixteen theme and view the page — do the titles show up when the default theme is active?
If so, it would be great if you could reach out to the theme author. They will likely have a much better idea of what’s happening since they are more familiar with their code than we are.
If the titles still do not display in the default WordPress theme, please take a screenshot of it in that state and share it with me here so I can check it out.
Thanks so much!
Geoff