Which time zone does the organizer of the event have?
Could you give us the raw source of that event?

BEGIN:VCALENDAR
PRODID:-//Inverse inc./SOGo 2.3.1//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;BYMONTH=3;BYDAY=-1SU
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
UID:7F72-55E44480-3-57E65F00
SUMMARY: XXXXX
CLASS:PUBLIC
X-SOGO-COMPONENT-CREATED-BY:xxxxx.xxxxx@hu-berlin.de
CREATED:20150831T121402Z
DTSTAMP:20150831T121402Z
LAST-MODIFIED:20150902T080147Z
DTSTART;TZID=Europe/Berlin:20151021T113000
DTEND;TZID=Europe/Berlin:20151021T130000
TRANSP:OPAQUE
ATTENDEE;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE;CN=XXXX XXXX:mailto:xxxx.xxxx@hu-berlin.de
ORGANIZER;SENT-BY="MAILTO:xxxxx.xxxxx@hu-berlin.de";CN=XXXXX XXXXX:mailto:xxxxx.xxxxx@hu-berlin.de
END:VEVENT
END:VCALENDAR

Please upgrade to SOGo Version 2.3.2.
I assume that is Bug 0003344

Forgot to mention:
After Upgrading the owner has to open and store this event.

We already use SOGo 2.3.2.
The event is still shifted 1 hour after the change.

Did the owner open and store this event since upgrading?

Yes. She did it today.

Suspending for now - provide more evidence if the bug is still relevant.