Thanks for your reply.
I already found that thread, it’s the first you find via Google when you search for this problem. 
But it didn’t contain any solution for me.
Here is one sample from the Google calendar export:
BEGIN:VEVENT DTSTART:20210519T090000Z DTEND:20210519T103000Z DTSTAMP:20210526T094343Z UID:xxxxxxxxxxxxxxxxxxxxxxx@google.com CREATED:20210517T093103Z DESCRIPTION: LAST-MODIFIED:20210517T093103Z LOCATION: SEQUENCE:0 STATUS:CONFIRMED SUMMARY:xxxxxxxxxxxxxxxxxxxxxxx TRANSP:OPAQUE END:VEVENT
EDIT:
This is the surrounding of all the events:
BEGIN:VCALENDAR PRODID:-//Google Inc//Google Calendar 70.9054//EN VERSION:2.0 CALSCALE:GREGORIAN METHOD:PUBLISH X-WR-CALNAME:xxxxxxxxxxxxxxxxxxxxxxx X-WR-TIMEZONE:Europe/Berlin **** HERE ARE THE EVENTS **** END:VCALENDAR