Everyone who's tried to get the roundcube caldav calendar plugin to work with ownclouds calendar storage probably knows already: Owncloud uses an old SabreDAV backend and the caldav calendar plugin won't download events.
Well, here's a temporary workaround.
1. Log in to your owncloud account
* go to your calendar, click on the calendar button in the top right
* right-click on the download link for your calendar and copy the URL
(it'll generally look like this:
http://your.server.here/apps/calendar/export.php?calid=1 - the number is the relevant part)
* retrieve the CalDAV URL by clicking on the globe icon
(generally in the form
http://your.server.here/apps/calendar/caldav.php/calendars/[yourusername]/default%20calendar )
2. Log in to your roundcube account
* Go to Settings->Calendar and add the CalDAV URL (with your credentials of course)
* Set the default view to "All Calendars"
* Go to Calendar Feeds and add the download link you got, but modify it a little:
http://[yourusername]:[yourpassword]@your.server.here/apps/calendar/export.php?calid=1
Caveat: The feed only seems to reload when specifically forced to, clicking the replication button won't actually replicate the feed. Toggling the feed off and back on (by clicking the "Switch Feed" button) does update it. Logging out and back in also does, though that's IMO less convenient.