Hi all, I have this problem.
I'm attempting to do a caldav put operation to modify an existing event adding one attendee but the server return me an error 500.
I'm doing this from a java client application using caldav4j library. I can successfully execute put operations on this event to modify other fields, but when I try to add an attendee error 500 is returned.
This is the request body of my call.

Code:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:Zimbra-Calendar-Provider
BEGIN:VTIMEZONE
TZID:Europe/Berlin
BEGIN:STANDARD
DTSTART:19710101T030000
TZOFFSETTO:+0100
TZOFFSETFROM:+0200
RRULE:FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTH=10;BYDAY=-1SU
TZNAME:CET
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:19710101T020000
TZOFFSETTO:+0200
TZOFFSETFROM:+0100
RRULE:FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTH=3;BYDAY=-1SU
TZNAME:CEST
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
DTSTAMP:20131216T102346Z
DTSTART;TZID=Europe/Berlin:20131218T080000
DTEND;TZID=Europe/Berlin:20131218T093000
SUMMARY:Prova partecipante
UID:e63f710c-0caf-4e75-b3fa-89e3740d467c
CLASS:PUBLIC
ATTENDEE;PARTSTAT=NEEDS-ACTION:mailto:test2@prova.it
END:VEVENT
END:VCALENDAR
Any suggestion is apreciated.
Thank you very much.