This address looks wrong to me.
The CalDAV address also includes the username. You can display the full adress in Nextcloud (Calendar → Calendar-Settings → Copy macOS CalDAV-Adress). Somethins like:
/remote.php/dav/principals/users/<username>/
However, the following problem remains:
# Troubleshooting
macOS does not support syncing CalDAV/CardDAV over non-encrypted
http://connections. Make sure you havehttps://enabled
Source:
But honestly, what you’re doing here is extremely careless. From another post, I gather that you also transfer your other data unencrypted (not just calendars). Your ISP[1] or other people on the Wi-Fi network can read your password and all your transmitted data.
At this point, you would be better off with Google, Microsoft or Apple Cloud in terms of data protection… Or buy a managed Nextcloud.
[1] This also includes everyone between your ISP and your server.