Impossible d'installer Nextcloud

Bonjour,
J’ai un petit soucis avec une installation Nextcloud : Nextcloud ne fonctionne pas, après installation, le message suivant apparais lors du chargement de l’url :

Internal Server Error
The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

Des infos sur le système :

{
"host": "Debian 9.5",
"kernel": "4.9.0-8-amd64",
"packages": {
    "yunohost": {
        "repo": "stable",
        "version": "3.1.0"
    },
    "yunohost-admin": {
        "repo": "stable",
        "version": "3.1.0"
    },
    "moulinette": {
        "repo": "stable",
        "version": "3.1.0"
    },
    "ssowat": {
        "repo": "stable",
        "version": "3.1.0"
    }
},
"backports": [],
"system": {},
"nginx": [
    "nginx: the configuration file /etc/nginx/nginx.conf syntax is ok",
    "nginx: configuration file /etc/nginx/nginx.conf test is successful"
],
"services": {
    "glances": "exited (enabled)",
    "nslcd": "running (enabled)",
    "metronome": "running (enabled)",
    "postfix": "exited (enabled)",
    "rspamd": "failed (enabled)",
    "yunohost-firewall": "exited (enabled)",
    "nginx": "running (enabled)",
    "php7.0-fpm": "running (enabled)",
    "dnsmasq": "running (enabled)",
    "fail2ban": "running (enabled)",
    "yunohost-api": "running (enabled)",
    "mysql": "running (enabled)",
    "avahi-daemon": "running (enabled)",
    "dovecot": "running (enabled)",
    "redis-server": "failed (enabled)",
    "slapd": "running (enabled)",
    "ssh": "running (enabled)"
},
"applications": {
    "phpmyadmin": "phpMyAdmin",
    "opensondage": "OpenSondage",
    "nextcloud": "Nextcloud"
},
"security": {
    "CVE-2017-5754": {
        "name": "meltdown",
        "vulnerable": false
    }
}

}

Autre soucis : depuis que nextcloud est installé des

slapd[8981]: <= mdb_substring_candidates: (sudoUser) not indexed

apparaissent dans /var/log/syslog

et enfin des

Can't monitor system: [Errno 1] Server monitoring is not enabled

appaissent dans l’interface d’administration web.

Par où commencer ?