A similar thing happens when a event reminder is popping up in Thunderbird.
When I press "Dismiss" the reminder dialog is not closing and the event is not dismissed.
But the change was actually submitted to the sogo calendar backend.
When I then close the reminder dialog and synchronize the calendar again, I get a warning popup "The item has recently been changed on the server ..."

That will usually happen when you DO NOT customize extensions.rdf.

Well, but that is not the case here.
I have modified extensions.rdf - if I hadn't done the modification correctly the changes would not be written to my calendar backend, but they are.
I see the changes in the web interface.

The problem seems to be that the changes are not synchronized with Lightning so after a change the calendar in the backend is changed but the local copy in Lightning is not changed.

when I set the reminder of an event in Lightning I see this error in the error console:

Lightning:Parsing failed for parts of the item (while this is considered to be a minor issue, we continue processing the item):
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:20191010T141154Z

LAST-MODIFIED:20191010T141238Z

DTSTAMP:20191010T141238Z

UID:93ac0153-02b3-4752-97e0-340d0fa3d711

SUMMARY:Neuer Termin

DTSTART;TZID=Europe/Berlin:20191010T161500

DTEND;TZID=Europe/Berlin:20191010T171500

TRANSP:OPAQUE

CLASS:PUBLIC

SEQUENCE:1

X-MOZ-GENERATION:2

BEGIN:VALARM

ACTION:DISPLAY

TRIGGER;VALUE=DURATION:PT0S

X-LIC-ERROR;X-LIC-ERRORTYPE=PARAMETER-VALUE-PARSE-ERROR:Got a VALUE parame

ter with an illegal type for property: VALUE=DURATION

DESCRIPTION:Mozilla Standardbeschreibung

END:VALARM

END:VEVENT

END:VCALENDAR

Lightning seems to have a problem with "TRIGGER;VALUE=DURATION:PT0S"
as reminder setting, but this seems to be correct syntax according to
https://www.kanzaki.com/docs/ical/trigger.html

I am using the Lightning which came with the Thunderbird installer (Version 6.2.9 for TB 60.9.0)

In TB 68.1.1 with the Sogo connector beta addon I see this error, when I create a new event, which is sent to the backend correctly, but does not appear in Lightning:

TypeError: window.acceptDialog is not a function
calendar-event-dialog.js:73:10
SIOnAccept chrome://sogo-connector/content/calendar/calendar-event-dialog.js:73
SIOnLoadHandler chrome://sogo-connector/content/calendar/calendar-event-dialog.js:10
_fireButtonEvent chrome://global/content/bindings/dialog.xml:433
_doButtonCommand chrome://global/content/bindings/dialog.xml:407
acceptDialog chrome://global/content/bindings/dialog.xml:77
oncommand chrome://calendar/content/calendar-event-dialog.xul:1

PUT https://sogo4.test.local/SOGo/dav/xyz/Calendar/personal/f9da59d5-d5a9-4fd4-bf94-1d104df68584.ics
[HTTP/1.1 201 Created 75ms]

Request URL:https://sogo4.test.local/SOGo/dav/xyz/Calendar/personal/f9da59d5-d5a9-4fd4-bf94-1d104df68584.ics
Request method:PUT
Remote address:10.3.69.13:443
Status code:
201
Version:HTTP/1.1

Response headers (230 B)
Raw headers
Connection
Keep-Alive
Content-Length
0
Content-Type
text/plain; charset=UTF-8
Date
Thu, 10 Oct 2019 14:25:57 GMT
ETag
"gcs00000000"
Keep-Alive
timeout=5, max=100
Server
Apache/2.4.38 (Debian)
Request headers (592 B)
Raw headers
Accept
text/xml
Accept-Charset
utf-8,;q=0.1
Accept-Encoding
gzip, deflate, br
Accept-Language
en-US,en;q=0.5
Authorization
Basic Zmx1OmNvOTE1OQ==
Cache-Control
no-cache
Connection
keep-alive
Content-Length
832
Content-Type
text/calendar; charset=utf-8
Host
sogo4.test.local
If-None-Match

Pragma
no-cache
User-Agent
Mozilla/5.0 (Windows; U; Windo…hy/0.7.6.666 Lightning/68.1.1

REPORT https://sogo4.test.local/SOGo/dav/xyz/Calendar/personal/
[HTTP/1.1 207 Multi-Status 7ms]

Request URL:https://sogo4.test.local/SOGo/dav/xyz/Calendar/personal/
Request method:REPORT
Remote address:10.3.69.13:443
Status code:
207
Version:HTTP/1.1

Response headers (258 B)
Raw headers
Cache-Control
no-cache
Connection
Keep-Alive
Content-Length
285
Content-Type
text/xml; charset="utf-8"
Date
Thu, 10 Oct 2019 14:25:57 GMT
Keep-Alive
timeout=5, max=99
Pragma
no-cache
Server
Apache/2.4.38 (Debian)
Request headers (542 B)
Raw headers
Accept
text/xml
Accept-Charset
utf-8,*;q=0.1
Accept-Encoding
gzip, deflate, br
Accept-Language
en-US,en;q=0.5
Authorization
Basic Zmx1OmNvOTE1OQ==
Cache-Control
no-cache
Connection
keep-alive
Content-Length
275
Content-Type
text/xml; charset=utf-8
Depth
1
Host
sogo4.test.local
Pragma
no-cache
User-Agent
Mozilla/5.0 (Windows; U; Windo…hy/0.7.6.666 Lightning/68.1.1

?

In TB 68 the same Lightning parsing errors appear as in TB 60:

TRIGGER;VALUE=DURATION:-PT15M

X-LIC-ERROR;X-LIC-ERRORTYPE=PARAMETER-VALUE-PARSE-ERROR:Got a VALUE parameter with an illegal type for property: VALUE=DURATION

I've fixed the

TypeError: window.acceptDialog is not a function

error.