Many thanks for your message.
Regarding the sidebar the problem remains with the blog page (http://www.airesprotegees.fr/blog-2/) where the option sidebar doesn’t work, even with default position.
I have contacted the support of ubermenu to make a wide sub-menu, it seems that there is a solution : “To create that submenu effect, you can use Unbounded Submenus with Rows. Note that you will need to set any wrappers from your theme to position:static – otherwise the submenus can only expand as wide as the next relatively positioned parent element from your theme.” As its related to the theme, do you have any idea of the steps to follow? Thanks for all!