I follow debian testing and a few month agos upgraded the kdepim suite to what seemed to be kf5,
kontact / kmail and korganizer show up as version 4:16.04.3.

My problem: I cannot any longer accept MS exchange meeting invitation emails which has worked for
years before.

The issue has two aspects.
FIRST - The display of the invitation message has changed. Where there were buttons to, e.g. Accept the
invitation, now there is simply nothing, white display surface, independent of checking the MS compatibilty options
in the kmail 'Misc' settings or of display as text or html. If I leave the display as html, the HTML message
status bar is initially black, but turns grey on left click , associated by a message on the console:

log_text_calendar: The top-level content is not a message. Cannot handle the invitation then.

Doesn't sound too good.
Message structure is

Content-Type: multipart/alternative;
boundary="[stuff deleted]"
MIME-Version: 1.0
[stuff deleted]
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: base64
[stuff deleted]
Content-Type: text/calendar; charset="utf-8"; method=REQUEST
Content-Transfer-Encoding: base64

SECOND My attempted workaround (trying to directly import the attached calendar item into korganizer fails miserably -
here is how:
- either: right click on calendar entry in the message structure display, korganizer component opens, select a calendar, claims to have successfully imported the item, nothing shows up in the week or month views
- or save the attachment first to disk, import into korganizer component, same thing happens.

What is remarkable is that if the item is imported into a cloud based resource (owncloud calendar), it gets uploaded to that server. The item not displaying is not
resource specific, it will also happen for filesystem based calendar resources. Also, removing/re-adding the resource does not let the calendar entries reappear.

Here's the calendar item, personal information removed:
BEGIN:VCALENDAR
METHOD:REQUEST
PRODID:Microsoft Exchange Server 2010
VERSION:2.0
BEGIN:VTIMEZONE
BEGIN:STANDARD
DTSTART:16010101T030000
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010101T020000
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
ORGANIZER;CN="HHH":MAILTO:HHH@ipm.fraunhofer.de
ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="RRR":MAILTO:RRR@ipm.fraunhofer.de
DESCRIPTION;LANGUAGE=de-DE:Besuch
SUMMARY;LANGUAGE=de-DE:Besuch
DTSTART;TZID=W. Europe Standard Time:20160928T090000
DTEND;TZID=W. Europe Standard Time:20160928T130000
UID:040000008200E00074C5B7101A82E00800000000F0F1BDF3BA14D201000000000000000
0100000003470F290B47AB649899021DFDE479605
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20160922T082346Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION;LANGUAGE=de-DE:SEM 1
X-MICROSOFT-CDO-APPT-SEQUENCE:0
X-MICROSOFT-CDO-OWNERAPPTID:-839477280
X-MICROSOFT-CDO-BUSYSTATUS:TENTATIVE
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INSTTYPE:0
X-MICROSOFT-DISALLOW-COUNTER:FALSE
BEGIN:VALARM
ACTION:DISPLAY
DESCRIPTION:REMINDER
TRIGGER;RELATED=START:-PT15M
END:VALARM
END:VEVENT
END:VCALENDAR

Any other ideas what I can try/configure/delete ?

Thanks!

I saw this issue with invitations, but I believe the problem was fixed in Applications 16.08 and above.