iCal calendar events no longer cancelling after outlook update - outlook

Question

I have a system that generates emails, including embedded iCalendar events, which are sent to the relevant people.
The event is created from a task.
The task can be edited, which in turn updates the meeting times etc.
The task can also be cancelled, which again updates the meeting, by cancelling it.
This had all been working fine for over a year.
But then about 6 weeks ago office updated, including a very noticeable update to outlook (the way meetings/events requests are displayed is drastically different now, so some significant work has gone on in that area by the outlook team) - the problem is this update has broken iCalendar event cancellations.
The initial meeting still works fine, as does editing. But when cancelling instead of the embedded iCalendar event doing it's thing, I now just get an attachment titled : "not supported calendar message.ics"
I am using PHPMailer to send the emails, and use the ical feature provided by this library to attach the generated iCalendar event to an email.
I have tried to debug as best I can, which isn't easy as they also seem to have removed any way to view full email source.
However I have tried to streamline the iCalendar event down to it's absolute barebones, with no luck.
So I have instructed PHPMailer to dump the full email. for debugging purposes - output below:
Event Creation :
Content-Type: multipart/alternative;
boundary="b1_ee3827bf9d86f19f6cb13cbcbc05bb33"
Content-Transfer-Encoding: 8bit^M
^M
This is a multi-part message in MIME format.
--b1_ee3827bf9d86f19f6cb13cbcbc05bb33
Content-Type: text/plain; charset=us-ascii
[test] testing iCal
--b1_ee3827bf9d86f19f6cb13cbcbc05bb33
Content-Type: text/html; charset=us-ascii
Details:</b></u><br /><i>Off-Site</i><br /><u><b>Technical Contact Name:</b></u> Test User<br /><u><b>Technical Contact Number:</b></u> 12345<br /><u><b>Technical Contact e-mail:</b></u> anothertest#test.com<br /><u><b>Work to be done:</b></u><br /><p>[test] testing iCal</p>
--b1_ee3827bf9d86f19f6cb13cbcbc05bb33
Content-Type: text/calendar; method=REQUEST; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 10.0 MIMEDIR//EN
VERSION:2.0
METHOD:REQUEST
BEGIN:VTIMEZONE
TZID:Europe/London
BEGIN:STANDARD
DTSTART:20091101T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10
TZOFFSETFROM:+0100
TZOFFSETTO:+0000
TZNAME:GMT
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:20090301T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3
TZOFFSETFROM:+0000
TZOFFSETTO:+0100
TZNAME:BST
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
LAST-MODIFIED:20190902T151649
UID:d7f4bed92a83499c81392678cd655aeb
DTSTAMP:20190902T151649
DTSTART;TZID="Europe/London":20190906T090000
DTEND;TZID="Europe/London":20190906T170000
TRANSP:OPAQUE
SEQUENCE:0
SUMMARY:"test task"
LOCATION:
STATUS:CONFIRMED
ORGANIZER;CN="xxxx#xxxx.com":MAILTO:xxxx#xxxx.com
ATTENDEE;CN="xxxx#xxxx.com";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED;MAILTO:xxxx#xxxx.com
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-DISALLOW-COUNTER:FALSE
X-MS-OLK-AUTOFILLLOCATION:FALSE
X-MS-OLK-CONFTYPE:0
CLASS:PUBLIC
PRIORITY:5
END:VEVENT
END:VCALENDAR
--b1_ee3827bf9d86f19f6cb13cbcbc05bb33--
Update event:
Content-Type: multipart/alternative;
boundary="b1_23a19fb87bb01c1c2ae6e3fede170549"
Content-Transfer-Encoding: 8bit^M
^M
This is a multi-part message in MIME format.
--b1_23a19fb87bb01c1c2ae6e3fede170549
Content-Type: text/plain; charset=us-ascii
[test] testing iCal - updated
--b1_23a19fb87bb01c1c2ae6e3fede170549
Content-Type: text/html; charset=us-ascii
<b>Delivery Details:</b></u><br /><i>Off-Site</i><br /><u><b>Technical Contact Name:</b></u> Test User<br /><u><b>Technical Contact Number:</b></u> 12345<br /><u><b>Technical Contact e-mail:</b></u> anothertest#test.com<br /><u><b>Work to be done:</b></u><br /><p>[test] testing iCal - updated</p>
--b1_23a19fb87bb01c1c2ae6e3fede170549
Content-Type: text/calendar; method=REQUEST; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 10.0 MIMEDIR//EN
VERSION:2.0
METHOD:REQUEST
BEGIN:VTIMEZONE
TZID:Europe/London
BEGIN:STANDARD
DTSTART:20091101T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10
TZOFFSETFROM:+0100
TZOFFSETTO:+0000
TZNAME:GMT
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:20090301T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3
TZOFFSETFROM:+0000
TZOFFSETTO:+0100
TZNAME:BST
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
CATEGORIES:Louis Marchant
LAST-MODIFIED:20190902T152651
UID:d7f4bed92a83499c81392678cd655aeb
DTSTAMP:20190902T152651
DTSTART;TZID="Europe/London":20190907T090000
DTEND;TZID="Europe/London":20190907T170000
TRANSP:OPAQUE
SEQUENCE:2
SUMMARY:"test event"
LOCATION:
STATUS:CONFIRMED
ORGANIZER;CN="xxxx#xxxx.com":MAILTO:xxxx#xxxx.com
ATTENDEE;CN="xxxx#xxxx.com";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED;MAILTO:xxxx#xxxx.com
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-DISALLOW-COUNTER:FALSE
X-MS-OLK-AUTOFILLLOCATION:FALSE
X-MS-OLK-CONFTYPE:0
CLASS:PUBLIC
PRIORITY:5
END:VEVENT
END:VCALENDAR
--b1_23a19fb87bb01c1c2ae6e3fede170549--
Cancellation event - the one no longer working :
Content-Type: multipart/alternative;
boundary="b1_87ddce1a469e89e0c0caa7b0a82efd5e"
Content-Transfer-Encoding: 8bit^M
^M
This is a multi-part message in MIME format.
--b1_87ddce1a469e89e0c0caa7b0a82efd5e
Content-Type: text/plain; charset=us-ascii
Meeting Cancelled
--b1_87ddce1a469e89e0c0caa7b0a82efd5e
Content-Type: text/html; charset=us-ascii
Meeting Cancelled
--b1_87ddce1a469e89e0c0caa7b0a82efd5e
Content-Type: text/calendar; method=REQUEST; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 10.0 MIMEDIR//EN
VERSION:2.0
METHOD:CANCEL
BEGIN:VTIMEZONE
TZID:Europe/London
BEGIN:STANDARD
DTSTART:20091101T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10
TZOFFSETFROM:+0100
TZOFFSETTO:+0000
TZNAME:GMT
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:20090301T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3
TZOFFSETFROM:+0000
TZOFFSETTO:+0100
TZNAME:BST
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
CATEGORIES:Louis Marchant
LAST-MODIFIED:20190902T152921
UID:d7f4bed92a83499c81392678cd655aeb
DTSTAMP:20190902T152921
DTSTART;TZID="Europe/London":20190907T090000
DTEND;TZID="Europe/London":20190907T170000
TRANSP:OPAQUE
SEQUENCE:2
SUMMARY:"Louis Marchant - Offsite - 2 Circles Communication Limited 602"
LOCATION:
STATUS:CANCELLED
ORGANIZER;CN="xxxx#xxxx.com":MAILTO:xxxx#xxxx.com
ATTENDEE;CN="xxxx#xxxx.com";CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED;MAILTO:xxxx#xxxx.com
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-DISALLOW-COUNTER:FALSE
X-MS-OLK-AUTOFILLLOCATION:FALSE
X-MS-OLK-CONFTYPE:0
CLASS:PUBLIC
PRIORITY:5
END:VEVENT
END:VCALENDAR
--b1_87ddce1a469e89e0c0caa7b0a82efd5e--
Any insights would be helpful, as I am fresh out of ideas. Although I suspect it may just simply no longer be possible, although I obviosuly hope this is not the case.

Answer 1

PHPMailer's iCal support has some limitations, some related to the horribly inconsistent way that it's supported in different clients. This issue discusses some of those problems.
A separate problem relates to iCal events other than REQUEST, specifically in Outlook. This problem is visible in your examples:
Content-Type: text/calendar; method=REQUEST; charset=iso-8859-1
METHOD:CANCEL
Notice that the two method names do not match; this is because PHPMailer currently only supports REQUEST-type messages, and it's likely that Outlook doesn't like this mismatch. What I suggested in that second issue was that PHPMailer could scan the content of the iCal data provided in the ICal property and extract the method value, and then use it in the content-type header used for that element.
PHPMailer doesn't currently do this - but if you'd care to add support for it and submit a pull request, it would be very welcome.

Answer 2

After outlook update I had the same issue. I was able to create event successfully but when i try to cancel the event, cal_event is not embedded within mail like how it was happening before. Instead sending the mail with an unsupported .ics attachment.
I got the solution for this. This worked for me.
Issue:
$mail->addCustomHeader('Content-type',"text/calendar; name=event.ics; method=REQUEST; charset=UTF-8;");
Change the method type from REQUEST to CANCEL
Resolution:
$mail->AddStringAttachment($ical, "event.ics", "7bit", "text/calendar; charset=utf-8; method=CANCEL");