[force_https not working on linux]

On Ubuntu Server 16.04.1 LTS (Xenial Xerus) I've installed Resilio Sync and started by:

systemctl enable resilio-sync

adjusted the /etc/resilio-sync/config.json file:

{
    "listening_port" : 0,
    "storage_path" : "/var/lib/resilio-sync/",
    "pid_file" : "/var/run/resilio-sync/sync.pid",
    "agree_to_EULA": "yes",

    "webui" :
    {
        "listen" : "0.0.0.0:999",
        "login" : "xxx",
        "password_hash" : "xxx"
    }
}

restarted the service, and everything works fine, on URL http://196.168.666.666:999/gui.

Next, I'm creating a certificate in /etc/resilio-sync/ folder:

openssl req -x509 -nodes -days 9999 -newkey rsa:2048 -keyout sync.key -out sync.crt

and updating config.json:

{
    "listening_port" : 0,
    "storage_path" : "/var/lib/resilio-sync/",
    "pid_file" : "/var/run/resilio-sync/sync.pid",
    "agree_to_EULA": "yes",

    "webui" :
    {
        "listen" : "0.0.0.0:999",
        "login" : "xxx",
        "password_hash" : "xxx"

        ,"force_https" : true
        ,"ssl_certificate" : "/etc/resilio-sync/sync.crt"
        ,"ssl_private_key" : "/etc/resilio-sync/sync.key"
    }
}

Unfortunately, now when I access https://196.168.666.666:999/gui the connection fails. Any idea what may be wrong?