ICS file in Outlook gives "The operation failed." only if I try to Accept/Tentative/Decline before saving

Question

We are creating an iCalendar .ics file in our system so our users can add an appointment to Outlook. A new requirement is that we have the Accept/Tentative/Decline options show up in Outlook along with RSVPs. I am able to achieve this by adding ATTENDEE and ORGANIZER lines to the .ics as follows:

ATTENDEE;RSVP=TRUE:
ORGANIZER;CN="Test Organizer":mailto:test@test.com

However, I'm running into a particular situation where Outlook returns "The operation failed." When I first open the .ics in Outlook, BEFORE it is actually saved into my calendar, if I click Accept/Tentative/Decline at that point, I get "The operation failed." If I do it AFTER it is saved into my calendar, then it works fine (RSVPs send if I delete it, and Accept/Tentative/Decline work exactly as expected). Here is video of that: http://screencast.com/t/BBUzYRwCJ

What am I doing wrong? Have I omitted something important from the .ics file? Here is the full file, which works great except for this one situation:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//ICSTestCS/
CALSCALE:GREGORIAN
BEGIN:VTIMEZONE
TZID:Europe/London
BEGIN:STANDARD
DTSTART:20071104T020000
RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=11
TZOFFSETFROM:0300
TZOFFSETTO:0200
TZNAME:GMT
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:20070311T020000
RRULE:FREQ=YEARLY;BYDAY=2SU;BYMONTH=3
TZOFFSETFROM:0200
TZOFFSETTO:0300
TZNAME:GMT
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
ATTENDEE;RSVP=TRUE:
ORGANIZER;CN="Test Organizer":mailto:test@test.com
DTSTART;TZID=Europe/London:20140522T090000
DTEND;TZID=Europe/London:20140522T103000
SUMMARY:Test Course
DESCRIPTION;ENCODING=QUOTED-PRINTABLE:Test Course Description
LOCATION:Test Office
UID:1
SEQUENCE:0
DTSTAMP:20140522T090000
END:VEVENT
END:VCALENDAR

Answer 1

I seem to have resolved my issue by including:

METHOD:REQUEST

at the top of the file under VERSION:2.0

Answer 2

You should have a mailto: uri as the ATTENDEE value, e.g.

ATTENDEE;RSVP=TRUE:someattendee@test.com