[Bug]: outlook calendar events response does not get processed by outlook

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When responding a calendar event sent from outlook calendar, the generated ics file makes outlook to reject it and replace it with a not supported calendar message.ics. The event on outlook calendar does not get confirmed.

original ics file sent by outlook calendar:

BEGIN:VCALENDAR
METHOD:REQUEST
PRODID:Microsoft Exchange Server 2010
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Romance Standard Time
BEGIN:STANDARD
DTSTART:16010101T030000
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010101T020000
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
ORGANIZER;CN=Miquel:mailto:someuser@hotmail.com
ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN=Miquel:mai
 lto:invited@example.com
DESCRIPTION;LANGUAGE=es-MX:\n..............................................
 ..........................................................................
 .................\nUnirse a una reunión en línea<https://join.skype.com/
 PH......E7AM>\n...........................................................
 ..........................................................................
 ....\n
UID:040000008...
SUMMARY;LANGUAGE=es-MX:Test resposta
DTSTART;TZID=Romance Standard Time:20231019T100000
DTEND;TZID=Romance Standard Time:20231019T103000
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20231018T161542Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION;LANGUAGE=es-MX:
X-MICROSOFT-CDO-APPT-SEQUENCE:0
X-MICROSOFT-CDO-OWNERAPPTID:2121963206
X-MICROSOFT-CDO-BUSYSTATUS:TENTATIVE
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INSTTYPE:0
X-MICROSOFT-ONLINEMEETINGEXTERNALLINK:https://join.skype.com/PH......E7AM
X-MICROSOFT-ONLINEMEETINGINFORMATION:{"OnlineMeetingChannelId":null\,"Onlin
 eMeetingProvider":2}
X-MICROSOFT-DONOTFORWARDMEETING:FALSE
X-MICROSOFT-DISALLOW-COUNTER:FALSE
X-MICROSOFT-REQUESTEDATTENDANCEMODE:DEFAULT
X-MICROSOFT-LOCATIONS:[]
BEGIN:VALARM
DESCRIPTION:REMINDER
TRIGGER;RELATED=START:-PT15M
ACTION:DISPLAY
END:VALARM
END:VEVENT
END:VCALENDAR

Response generated by nextcloud calendar:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Sabre//Sabre VObject 4.4.2//EN
CALSCALE:GREGORIAN
METHOD:REPLY
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
UID:040000008...
DTSTAMP:20231018T173746Z
SEQUENCE:0
DTSTART;TZID=Romance Standard Time:20231019T110000
DTEND;TZID=Romance Standard Time:20231019T120000
SUMMARY:Test 1
ORGANIZER;CN=Miquel:mailto:someuser@hotmail.com
ATTENDEE;PARTSTAT=ACCEPTED;CN=Miquel:mailto:invited@example.com
END:VEVENT
END:VCALENDAR

Seems that removing TZID on DTSTART and DTEND makes the ics file valid.
Also noted that when Gmail is responding to an outlook calendar event, it's sending DTSTART and DTEND similar to isostring:

DTSTART:20231019T090000Z
DTEND:20231019T100000Z

Also Gmail is adding the X-MICROSOFT-CDO fields and does not include the BEGIN:VTIMEZONE, BEGIN:DAYLIGHT and BEGIN:STANDARD.
Inside BEGIN:VEVENT section Gmail also adds a STATUS:CONFIRMED field.

Also noted that the email sent by Gmail in outlook appears with an icon on the email marking as a calendar email. When responding from nextcloud, the calendar icon never appears. I think it might be because of the headers of the email, but I could not inspect the headers and compare to the one sent by nextcloud...

Steps to reproduce

1. Create an event with calendar from hotmail.com
2. Add the event to nextcloud calendar importing ics file in calendar (or via snappymail calendar connector)
3. Go to calendar and mark the calendar meeting as accepted
4. Go to outlook web and check the email received from nextcloud calendar response

Expected behavior

Responding to an outlook calendar event should make the event in outlook calendar to process the action (accept/tentative/reject)

Installation method

Community Docker image

Nextcloud Server version

27

Operating system

Debian/Ubuntu

PHP engine version

PHP 7.4

Web server

Nginx

Database engine version

PostgreSQL

Is this bug present after an update or on a fresh install?

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by reproducing the Outlook response flow described in the issue and compare the original invitation ICS with Nextcloud's generated REPLY. Investigate the generated DTSTART/DTEND timezone data and email response format; done means Outlook processes accept, tentative, and reject responses instead of replacing them with an unsupported calendar message.