curl -I http://sesstim4.timone.univ-amu.fr -A "Let's Encrypt validation server"
You did not include the full path, nor the full header.
Try:
curl -Ii http://sesstim4.timone.univ-amu.fr/.well-known/acme-challenge/sometestfile \
-A "Mozilla/5.0 (compatible; Let's Encrypt validation server; +https://www.letsencrypt.org)"