I’ve installed the MacOS client app. (https://nextcloud.com/install/#install-clients)
Out of the box the Mac client runs a wizard to get you connected.
The URL suggested by https://office.coops.tech is https://office.coops.tech/remote.php/webdav/
The Mac client responds to this with a “Connection Failed” message, suggesting that I should either use a different URL, retry over unencrypted http, or configure a client-side TLS certificate. Then reports that the SSL handshake failed.
Browsing to this URL offers a login, and if I use my Nextcloud login credentials it gets me into a page that reports that this is the WebDAV interface, suggesting that my client config is at fault.
Digging into this it transpires that this client is broken and doesn’t work. Mac users should instead download the current beta version (18.104.22.168 as I write this) from https://download.nextcloud.com/desktop/prereleases/Mac/
Why they have a public release of the Mac client that doesn’t actually work seems pretty weird to me, but this beta release seems to work fine for me.