ludo974
Hi
in Joomla if an article or category does not associated to a menu, it will open under homepage.
The category Témoignages clients is not associated to any menu item. so when you click on the articles it will open under homepage and home menu does not have mainbody section to display output.
Kindly create a menu with type > category blog > assign Témoignages clients into this menu and use default template style.
Now when you open the article under this category it will open under the menu you added.
Regards