I have not tried the OS X server but yes, you most likely need to define some path to the url. Apple's own products might use some non-standard way to figure out the calendar path but the server's logs will probably give some idea what the correct url should be. You can also run Rainlendar in the debug mode and check from the debug log
what exactly the server is returning to Rainlendar.
If the basic authentication does not work you can try the digest authentication (I think iCloud uses that one too).