I am currently evaluating em Client and I am running into the following problem.
We are using a Horde Groupware instance to synchronize calendars and address books via CalDav and CardDav.
After setting up a Calendar/Contact account and point it at the horde server http://mail.server.com/rpc.php/principals// the calendars and address books are correctly discovered (and the address books work fine), but as soon as em Client start synchronizing we get the following error:
15:24:28 [CalDAV / CardDAV] Synchronisiere Elemente: Die Synchronisation vom Element /rpc.php/calendars//calendar:LivF4OKsiACgZju4Bd317w4/b90701d9-d92d-4dd9-9d24-965f193e43b0-caldavsyncadapter.ics ist fehlgeschlagen, durch folgenden Fehler: DTSTART
The error message is in German so the gist of it is
Synchronizing Elements: The Synchronization of Element … failed, because of error: DTSTART
So the entries in the calendar get sync back to the server but only a few entries are synchronized down. Our mobile clients, mostly Android based synchronize just fine also via CalDav.
Also task seem to work right out of the box.
Our Horde setup is v5.2.3 with kronolith at 4.2.3 an we use em Client 6.0.21040
If you need any more informations, please let me know.