I can't reproduce the issue.

When was the event created?

Was it a repetitive event?

Show your full list of installed SOGo/SOPE packages.

Thanks for your quick answer.

The bug can be triggered when the event, is, for example created today
for an event tomorrow.

As an example, I just created in TB an event at 2PM tomorrow.

Here is the dump of this event from the SOGo database:

BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Europe/Paris
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
CREATED:20191113T165611Z
LAST-MODIFIED:20191113T165620Z
DTSTAMP:20191113T165620Z
UID:936c8c01-4a97-4037-850a-83bdf1fa5524
SUMMARY:Event starting at 2PM created in TB
DTSTART;TZID=Europe/Paris:20191115T140000
DTEND;TZID=Europe/Paris:20191115T150000
TRANSP:OPAQUE
CLASS:PUBLIC
X-MOZ-GENERATION:1
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER;VALUE=DURATION:-PT15M
DESCRIPTION:Default Mozilla Description
X-LIC-ERROR;X-LIC-ERRORTYPE=PARAMETER-VALUE-PARSE-ERROR:Got a VALUE paramet
er with an illegal type for property: VALUE=DURATION
END:VALARM
END:VEVENT
END:VCALENDAR

It was not a repetitive event.

It appears starting at 1PM instead of 2PM on my Smartphone which is
connected through ActiveSync. Also in the web interface of SOGo it
appears as starting at 1.00PM

My full package list is below:
libsbjson2.3_4.9.r1664.20191108_amd64.deb
libsbjson2.3-dev_4.9.r1664.20191108_amd64.deb
libsope4.9-dev_4.9.r1664.20191108_all.deb
libsope-appserver4.9_4.9.r1664.20191108_amd64.deb
libsope-appserver4.9-dev_4.9.r1664.20191108_amd64.deb
libsope-core4.9_4.9.r1664.20191108_amd64.deb
libsope-core4.9-dev_4.9.r1664.20191108_amd64.deb
libsope-gdl1-4.9_4.9.r1664.20191108_amd64.deb
libsope-gdl1-4.9-dev_4.9.r1664.20191108_amd64.deb
libsope-ldap4.9_4.9.r1664.20191108_amd64.deb
libsope-ldap4.9-dev_4.9.r1664.20191108_amd64.deb
libsope-mime4.9_4.9.r1664.20191108_amd64.deb
libsope-mime4.9-dev_4.9.r1664.20191108_amd64.deb
libsope-xml4.9_4.9.r1664.20191108_amd64.deb
libsope-xml4.9-dev_4.9.r1664.20191108_amd64.deb
libwbxml2-0_0.11.6-1_amd64.deb
libwbxml2-0-dbg_0.11.6-1_amd64.deb
libwbxml2-dev_0.11.6-1_amd64.deb
libwbxml2-utils_0.11.6-1_amd64.deb
sogo_4.1.1.20191112-1_amd64.deb
sogo-activesync_4.1.1.20191112-1_amd64.deb
sogo-dbg_4.1.1.20191112-1_amd64.deb
sogo-dev_4.1.1.20191112-1_amd64.deb
sope4.9-appserver_4.9.r1664.20191108_amd64.deb
sope4.9-dbg_4.9.r1664.20191108_amd64.deb
sope4.9-gdl1-mysql_4.9.r1664.20191108_amd64.deb
sope4.9-gdl1-postgresql_4.9.r1664.20191108_amd64.deb
sope4.9-libxmlsaxdriver_4.9.r1664.20191108_amd64.deb
sope4.9-stxsaxdriver_4.9.r1664.20191108_amd64.deb

And what is the value of the SOGoTimeZone parameter in your sogo.conf?

What is the value of the user's timezone parameter from the General preferences panel?

root@server:~# cat /etc/sogo/sogo.conf | grep SOGoTimeZone
/ General - SOGoTimeZone MUST be defined /
SOGoTimeZone = Europe/Paris;

and the user timezone is also Europe/Paris.

Good evening,
I just wanted to check with you if you need any extra further information/test to allow you to have a look at this unexpected behavior/bug.
This bug is critical for our organization and any hint on how to solve it would be greatly appreciated.
Best regards.

Please can you check this bug? I can supply new information if needed. Best regards.

This might be similar to what is described in bug 4894.
Please provide the same Infos requested in bug 4894.

Please try the latest nightly build.

I installed the latest nightly build and I can confirm that the problem is solved with the nightly build (tested with sogo_4.1.1.20191215-1_amd64.deb etc ...).
Big thanks to Francis and Ludovic for their feedback.
Have great day.

This ticket can be CLOSED and marked as SOLVED.