ICS files won't import in Thunderbird (and Outlook)

We have this ICS file (exported from a test event) which won't import properly in Thunderbird (and Outlook):

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Plone.org//NONSGML plone.app.event//EN
X-WR-TIMEZONE:UTC
BEGIN:VEVENT
SUMMARY:Test Termin
DTSTART;VALUE=DATE:20210602
DTEND;VALUE=DATE:20210605
DTSTAMP;VALUE=DATE-TIME:20210608T072717Z
UID:b38dfbb62ead4ef9949fb73a47577b03@test.xxx.de
CREATED;VALUE=DATE-TIME:20210602T134537Z
DESCRIPTION:Test Termin
LAST-MODIFIED;VALUE=DATE-TIME:20210602T134553Z
URL:https://test.xxx.de/de/unternehmen/test-termin
END:VEVENT
END:VCALENDAR

Summary, description and URL are missing.

The import works fine with Google Calendar or on Mac.

Any idea?

perhaps a datetime format problem? UTC and the Z at the end of the `DTSTAMP``?

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