Adding and deleting from French menu

Good morning,
We have been using wordpress for our library website (csllibrary.org) for over two years and have recently encountered a problem when trying to add or delete items from our French menu. This problem does not occur with the English menu. When we try with the french version (this is a bilingual site because of our province), we are redirected to an error page that indicates “page not found/ error 404- not found”. See here:
https://csllibrary.org/wp-admin/undefined?lang=fr
We attempted to deactivate all of our plugins and remove ones that were inactive, but this did not solve the problem. Any help would be appreciated.
Thank you.