Maybe you can try entering https://example.com/webdav/user in your browser to see whether it works. Seems like a SSL error, and maybe you can check your network (try different WiFi / mobile data) and the server SSL configuration.
SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure
It works fine in the browser, as far as I can tell (see also first paragraph of my post above for a bunch of other urls that work).
Also when I curl:$ curl -i https://user:firstname.lastname@example.org/webdav/user/caldav HTTP/1.1 200 OK Server: nginx/1.6.2 Date: Tue, 21 Mar 2017 19:53:43 GMT Content-Type: text/calendar Content-Length: 90 Connection: keep-alive Last-Modified: Tue, 21 Mar 2017 11:24:31 +0000 ETag: "b51573a597a4ec9d6ad3df29271c171e" Strict-Transport-Security: max-age=15768000 BEGIN:VCALENDAR PRODID:-//Radicale//NONSGML Radicale Server//EN VERSION:2.0 END:VCALENDAR