I just tried connecting my Nautilus (file manager) to a davros grain via webdav. The Server Name provided on the Clients tab looks like https://api-43d8ad9c4cc255113b03bcd7b919c772.sandstorm.stephentpollard.com/. When I copied that, changed the https to davs, and put it into my nautilus, and copied over the credentials, it still did not connect.
I remembered '/remote.php/webdav' from the client side code and decided to append that to the url provided. That worked like a charm.
I think there should be a mention on the client tab that says to try appending remote.php/webdav to your url if you're having trouble connecting. Perhaps the owncloud/nextcloud clients automatically append that, but not all clients do.