I know it is an old thread but for someone who is looking for more answers as I did: I have self hosting Nextcloud instance on Apache using Raspberry Pi 3 and I have generated self-signed certificate for it. The eOS calendar refused to connect at all. I had to generate the certificate through Let’s Encrypt and now it’s working flawlessly.
But as far as I know you have to add each individual calender separately. In OwnCloud / NextCloud each individual calendar has a link button, maybe behind a submenu (3 dots). The link that appears is the link you use as URL. Then when you also include the correct username and press Create calendar it should ask for your password.
I'm sure I've read this on a OwnCloud page somewhere, but couldn't find it anymore.