I delete the file and i force a regen_conf of mdns. the problem is indeed linked to rights:
mv /etc/yunohost/mdns.yml /etc/yunohost/mdns.yml.old
yunohost tools regen-conf mdns --force
ls -la /etc/yunohost/mdns.yml
-rw-r--r-- 1 mdns mdns 30 Mar 14 14:20 /etc/yunohost/mdns.yml
Now, you can restart the yunomdns service and delete /etc/yunohost/mdns.yml.old if all is allright.