You use the wrong URL.
Don't use the ICS one.
Instead use the CalDAV URL in Thunderbird/Lightning.

Sorry i don't understand why i use the wrong URL. I mark the event in Thunderbird/Lightning an delete it and nothing happens.

You added the whole calendar in Lightning with the wrong URL.
ICS-URLs are read only.
So can not change anything in this calendar.

Please drop this calendar and recreate it with the CalDAV URL.

We add the URL automated with the SOGO Plugins and not manually.

The resulting address is the following:
https://domain.de/SOGo/dav/gunnarweissmann/Calendar/personal/

We tested the issue again with an existing and a new event and get the following results:

deleting an old repeating event (didn't work):
"PUT /SOGo/dav/gunnarweissmann/Calendar/personal/0a2969db-21c1-4a8a-ac6a-5b26c74a6ea0.ics HTTP/1.1" 403 396/1915 1.550 - - 0
deleting new created repeating event (work):
"PUT /SOGo/dav/gunnarweissmann/Calendar/personal/95bdd701-5cab-f146-90e7-93fe5d58f542.ics HTTP/1.1" 204 0/955 0.269 - - 0

This behavior didn't exist with SOGo Version 2.2.9a.

When we still use the wrong URL could you please specify the right URL we have to use? We can't easily change the URL because we use the SOGO Plugins.

Thank you.

You are right.
My fault, I didn't see the /personal/ part.
So I thought you got the wrong calendar URL in your Lightning settings.

Is the "immortal" event one of an event series you own, or one you were invited to?

The event is my own event. The issue is reproducible for others users too.

Then it's not a permission problem.
Because it is your calendar and your event.

I think so too because it is possible to delete the event in the webinterface.

What can we do to figure out the problem?

Do you get any errors in Thunderbirds error console?

yes this one:

[JavaScript Error: "CalDAV: Unexpected status modifying item to Gunnar Weissmann: 403
BEGIN:VCALENDAR

PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN

VERSION:2.0

BEGIN:VTIMEZONE

TZID:Europe/Berlin

X-LIC-LOCATION: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:20140923T084107Z

LAST-MODIFIED:20141219T091743Z

DTSTAMP:20141219T091743Z

UID:8365fb6d-92e6-cc49-a7ef-49ff0c0ec4bd

SUMMARY:Two Player: CoOp

ORGANIZER;RSVP=TRUE;CN=Gunnar Weissmann;PARTSTAT=ACCEPTED;ROLE=CHAIR:mailt

o:gunnar.weissmann@doccheck.com

RRULE:FREQ=WEEKLY

EXDATE:20141209T140000Z

EXDATE:20141216T140000Z

EXDATE:20141223T140000Z

EXDATE:20141230T140000Z

CATEGORIES:Interner Termin

DTSTART;TZID=Europe/Berlin:20140923T150000

DTEND;TZID=Europe/Berlin:20140923T163000

CLASS:PUBLIC

X-MOZ-GENERATION:6

DESCRIPTION:

TRANSP:OPAQUE

SEQUENCE:8

END:VEVENT

END:VCALENDAR

"]

Is this information useful for debugging?

I've tested with the event above and it works for me.

I guess we would need to look at this "live" on the system. But for this, you would require a support contract:

http://inverse.ca/support.html

No feedback provided in many months, suspending.