Clean hassio install but not secure duckdns adress

Configuration
1

I just got my 4. clean install of hassio on my RP3 today. The first add-on I install is Duckdns.
I install it as described in add-on and change only the token and url (using the same token and sub domain I have used before (from duckdns.org page)).

Config in add-on:

{
  "lets_encrypt": {
    "accept_terms": false,
    "certfile": "fullchain.pem",
    "keyfile": "privkey.pem"
  },
  "token": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
  "domains": [
    "my-domain.duckdns.org"
  ],
  "seconds": 300
}

I’ve tried “accept_terms”: true …and false

Log looks fine…
ha2

the page does not appear as secure

ha1
ha1775×186 22.5 KB

Although there is nothing to say that you need to configure configuration.yaml, I have tried to add:

http:
  base_url: my-domain.duckdns.org:8123  
  ssl_certificate: /ssl/fullchain.pem
  ssl_key: /ssl/privkey.pem

both with and without https:// in base_url.

Add-on log then says:

# INFO: Using main config file /data/workdir/config
Processing *my-domain*.duckdns.org
 + Checking domain name(s) of existing cert... unchanged.
 + Checking expire date of existing cert...
 + Valid till Mar 12 11:36:52 2020 GMT Certificate will not expire
(Longer than 30 days). Skipping renew!

If I restart Hassio with these lines in configuration.yaml, it never starts up again and I have to reinstall.
What is happening here. Why is not the page secure?