I am having difficulty getting Owncloud calendar sync to work on OTA 13, which should support it directly.
I have a known-good Owncloud installation on an Ubuntu server, which syncs with several clients already. My account on that server has two calendars, which show as "Default2" and "Social events" in the web UI of the server. The CalDAV links which Owncloud gives for the calendar are respectively https://mydomain/owncloud/remote.php/dav/calendars/myname/default2/ and https://mydomain/owncloud/remote.php/dav/calendars/myname/socialevents/
I have a BQ Aquaris 4.5 which I upgraded to OTA 13, then reset to default configuration. This gave me one calendar, "Personal", on the phone itself. I then added the Owncloud account, using a URL of the form https://mydomain/owncloud (substituting my domain name of course) and with the appropriate user name and password. The phone accepted this as correct. Still working in "Accounts", I enabled access to this account for the Calendar application.
At this point the Calendar now shows two calendars: the original "Personal" and a new "Birthdays & Anniversaries". The new calendar seems to be read-only and does not contain any events. The name does not correspond to the name of either calendar on the server. Synchronising the Calendar app flashes up a brief message
Sync done: (user account details)
which seems to indicate success. I can't find any way to show the real server calendars, or to establish that the Calendar app knows that they exist.