Tine2.0
Tine 2.0 is an open source groupware solution.
Setup
CalDAV-Sync
Tine 2.0 supports CalDAV since version Milan. Please note that support for task sync via CalDAV has been added in Collin.
CardDAV-Sync
Tine 2.0 supports CalDAV since version Milan.
Known Issues
The latest release of Tine 2.0 (Collin) has a bug when it comes to shared calendars, see 0009204: "Foreign" events won't sync/show up via CalDAV.. Users don't see events in shared calendars unless they are an attendee of the event.
To workaround that issue, either change the file Calendar/Frontend/WebDAV/Container.php
as mentioned in the bug report or enable the option Synology Workaround (in newer releases this option is called Use compatible sync method) in the CalDAV-Sync account settings. Using the compatible sync method will cause more traffic and higher battery drain, so you probably want to try the first way first.