This seems like a bug to me. I will move the topic to our #bug category.
To fix the issue on my site, I went to the Rails console and ran:
c = Category.find_by(name: 'exemple de catégorie')
c.update(slug: 'exemple-de-categorie')
After doing that, the category loads correctly. To make this change on your site, you’ll need to substitute your category’s name for the name I used in the above commands. Set the slug in a similar way to what I did above - replace spaces with - and remove any accented characters.
Let us know if you are not sure about how to access your site’s Rails console.