Comment 69 for bug 1846423

Now I did some more testing...
Because I have these problems only with a Nextcloud "Birthdays from contacts"-calendar, I setupped a new Nextcloud instance with only a few contacts with a birthday in it. This calendar is in Nextcloud created automatically and write protected for all users. The events in this calendar have a reminder, that's timed for midnight and are for the whole day. For comparing I created also a "normal" calendar, with and without reminder settings in the events, with special durations and for the whole day. So the normal Calendar works like it should. the reminder pops up and is closeable with the close button in the window and doesn't throw an error. The birthday calendar reminders are only closable over the X in the titlebar an throw the error. The "illegal type for property: VALUE=DURATION"-error doesn't show up in no case anymore. So I think its for me another problem now and your patch does what it should.

Lightning: CalDAV: Unexpected status modifying item to geburtstag aidlec: 404
BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VEVENT
CREATED:20210718T071933Z
LAST-MODIFIED:20210718T071947Z
DTSTAMP:20210718T071947Z
UID:6f28a0d9-f539-4181-841a-05d7280a61a2
SUMMARY:🎂 Jul Birthday (1976)
RRULE:FREQ=YEARLY
X-MOZ-LASTACK:20210718T071947Z
DTSTART;VALUE=DATE:19760717
DTEND;VALUE=DATE:19760718
TRANSP:TRANSPARENT
X-NEXTCLOUD-BC-FIELD-TYPE:BDAY
X-NEXTCLOUD-BC-UNKNOWN-YEAR:0
X-NEXTCLOUD-BC-YEAR:1976
X-MOZ-SNOOZE-TIME-1626480000000000:20210718T072447Z
X-MOZ-GENERATION:1
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER;VALUE=DURATION:PT0S
DESCRIPTION:🎂 Jul Birthday (1976)
END:VALARM
END:VEVENT
END:VCALENDAR

I'll do some more testing also with my productive data, which has around 900 contacts with 300 birthdays, an about 5 calendars with plenty of events. :) so finally it looks like the birthday events are although the error shown in the calendar, whats for me definitely is an improvement. ;)