CalDAV send no invitation to attendee by a special client (DAVx)

Hello,

I have two CalDAV client:

Thunderbird + Lightning on Linux (TB)
DAVx^5 on Android 7.0 (DAVx)

When I create a appointment and add some attendee with TB, then the server send a invitation mail to the attendee. When I do this via DAVx, the server does not create a invitation mail to the attendee.

Who can I debug this? Where is the problem?

I look to the ics data, but I can't see real differences:

ics data from Thunderbird:

BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Europe/Berlin
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=3
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
CREATED:20191209T202440Z
LAST-MODIFIED:20191209T202519Z
DTSTAMP:20191209T202519Z
UID:bc573bb9-9b1c-41f8-b55c-e82ad50fa89f
SUMMARY:Test TB 753951
ORGANIZER;RSVP=TRUE;PARTSTAT=ACCEPTED;ROLE=CHAIR:mailto:Weide****@g**.de
ATTENDEE;RSVP=TRUE;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT:mailto:m*****@ju*****.de
DTSTART;TZID=Europe/Berlin:20191210T220000
DTEND;TZID=Europe/Berlin:20191210T230000
TRANSP:OPAQUE
LOCATION:Test
END:VEVENT
END:VCALENDAR

and here the ics data from DAVx:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:+//IDN bitfire.at//DAVx5/2.6.1.1-gplay ical4j/2.2.6
BEGIN:VEVENT
DTSTAMP:20191208T201112Z
UID:02c3a1fd-adcd-4eba-bb16-f80f2abd41ed
SUMMARY:Test aCalendar 8965
LOCATION:Test
DTSTART;TZID=Europe/Berlin:20191215T100000
DTEND;TZID=Europe/Berlin:20191215T110000
STATUS:CONFIRMED
ORGANIZER:mailto:Weide****@g**.de
ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;RSVP=TRUE;PARTSTAT=NEEDS-AC
 TION:mailto:m*****@ju*****.de
ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;RSVP=TRUE;PARTSTAT=ACCEPTED
 :mailto:Weide****@g**.de
END:VEVENT
BEGIN:VTIMEZONE
TZID:Europe/Berlin
TZURL:http://tzurl.org/zoneinfo/Europe/Berlin
X-LIC-LOCATION:Europe/Berlin
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19810329T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19961027T030000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
END:STANDARD
BEGIN:STANDARD
TZOFFSETFROM:+005328
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:18930401T000000
RDATE:18930401T000000
END:STANDARD
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19160430T230000
RDATE:19160430T230000
RDATE:19170416T020000
RDATE:19180415T020000
RDATE:19400401T020000
RDATE:19430329T020000
RDATE:19440403T020000
RDATE:19450402T020000
RDATE:19460414T020000
RDATE:19470406T030000
RDATE:19480418T020000
RDATE:19490410T020000
RDATE:19800406T020000
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19161001T010000
RDATE:19161001T010000
RDATE:19170917T030000
RDATE:19180916T030000
RDATE:19421102T030000
RDATE:19431004T030000
RDATE:19441002T030000
RDATE:19451118T030000
RDATE:19461007T030000
RDATE:19471005T030000
RDATE:19481003T030000
RDATE:19491002T030000
RDATE:19800928T030000
RDATE:19810927T030000
RDATE:19820926T030000
RDATE:19830925T030000
RDATE:19840930T030000
RDATE:19850929T030000
RDATE:19860928T030000
RDATE:19870927T030000
RDATE:19880925T030000
RDATE:19890924T030000
RDATE:19900930T030000
RDATE:19910929T030000
RDATE:19920927T030000
RDATE:19930926T030000
RDATE:19940925T030000
RDATE:19950924T030000
END:STANDARD
BEGIN:DAYLIGHT
TZOFFSETFROM:+0200
TZOFFSETTO:+0300
TZNAME:CEMT
DTSTART:19450524T010000
RDATE:19450524T010000
RDATE:19470511T020000
END:DAYLIGHT
BEGIN:DAYLIGHT
TZOFFSETFROM:+0300
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19450924T030000
RDATE:19450924T030000
RDATE:19470629T030000
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0100
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19460101T000000
RDATE:19460101T000000
RDATE:19800101T000000
END:STANDARD
END:VTIMEZONE
END:VCALENDAR