Synology is a manufacturer of NAS devices. Newer firmware releases support CalDAV - in a way (see below).
CalDAV-Sync has been reported to work well with Syncology DiskStations that support CalDAV. But please be aware of the known issues below!
Please refer to the manual/configuration of your DiskStation for the correct path name. In general it should be sufficient to enter the path to your CalDAV root.
Important: A user reported that the path is case-sensitive.
After the account setup enable Use WebDAV sync method (in some translations called Synology Workaround) in the account settings. In some cases this is required to sync all events, see known issues below.
As of the new release of the DiskStation Manager (v5.1) the server also support the synchronisation via CardDAV. Just like for CalDAV, please refer to the manual / configuration of your setup.
Make sure that the user has WebDAV permissions on the station.
Here is an installation guide (in German). Make sure to add the DDNS address to config.php when you're using this feature.
There is a bug in Synology's CalDAV server. CalDAV-Sync uses calendar-query to get all events in the configured sync-window from the server. However, if there are no events in the sync-window the Synology firmware responds with an error "404 Not Found" instead of an empty list. This error will cause CalDAV-sync to cancel the synchronization. The only known workarounds at this time are to configure a larger sync-range in CalDAV-Sync's settings and/or to ensure that you always have at least one event in this range (you have to create them using another client that doesn't use calendar-query).
Another reason for 404 errors has been identified: This error can occur if a calendar contains sub-folders (beside the .DAV folder).
Some users have reported that their events disappear from the Android calendar app after being synced. This is caused by a bug in Synology's CalDAV implementation, but it's still unclear what triggers this behavior. One user reported syncing worked well after exporting the entire calendar, removing and recreating the calendar on the DiscStation and re-importing all events. Other users reported that didn't fix it. If you find a way to trigger this issue on a working calendar, please let me know. I'd like to report this issue to the folks at Synology.
Update: We discovered that the server cuts off the response to the request send by the app. That means the server simply doesn't return all events. That affects only the calendar-query request. The regular propfind request still returns all results (which explains why other clients still work in that case). In the tested case the server cut off the results after 91 events. It's still unclear whether it's a hard limit, a memory limit or a specific event that triggers the bug.
Update 2: We've implemented a workaround for Synology servers. You can enable it in the account settings. The workaround will use a WebDAV request to get the event list from the server. This method appears to fix it, but it is much less efficient.
Meanwhile you should consider to install OwnCloud on your Synology. The latest releases work well on Synology DiscStations.
Task support is broken on Synology DiscStations. The server returns an error 405 (Method not allowed) when the app tries to sync. Please disable task sync if you're syncing to a Synology DS.
Update: One user reported that this issue has been fixed in the Synology firmware version 4.3. Please report.
Synology does not support invitations.
Connection via HTTPS fails
According to Synology's support team, there is a problem with their implementation of HTTPS/2. They're working on the issue, but as a temporary workaround you can disable HTTPS/2 in Control Panel settings. (Thanks to G.Müller)