MacOS High Sierra 10.13.6
I have the cert, and set it to be always trusted in any case in the keychain.
Assuming that the proper url is the one given to me by the Calendar app in my NC instance, I still fail in both manual and automatic mode… 
Please note, I tried this both on my Macbook Pro and my Mac Mini; both have the same MacOS as mentioned above. Both use, successfully, the nextcloud app.
Adding CardDav also does not work, by the way…
