I do know that this is a bug from Apple. But unfortunately, I cannot resolve it by emailing them or whatever (it was not my decision to go with a big SV company).
Upon parsing the next line of X-APPLE-STRUCTURED-LOCATION
Error at line 3,736: Expected , read 
“t\n10116 Offenbach am Main”
Somehow thunderbird display this event properly (but my thunderbird is read-only).
Inside apple-calendar (both desktop app and WWW) this event does NOT exist. Otherwise I would delete it.
Is there any workaround (other than me deleting this calendar)?
Thanks for supporting open source!
BEGIN:VEVENT DTEND;TZID=Europe/Berlin:20171127T153000 UID:73BGAFCB-E708-4720-97EB-E85D715E16CD DTSTAMP:20171031T140858Z LOCATION:Goethe Str. 9\; Wis\n10116 Offenbach am Main | GERMA NY URL;VALUE=URI:message:%3C11bf01b45177email@example.com .de%3E SEQUENCE:0 SUMMARY:Hochschullehrer-Treffen DTSTART;TZID=Europe/Berlin:20171127T140000 CREATED:20171031T140815Z X-APPLE-STRUCTURED-LOCATION;VALUE=URI;X-TITLE=Max-von-Laue-Str. 9; Dekana t\n10116 Offenbach am Main | GERMANY:geo:50.373850,8.478694 END:VEVENT
If thunderbird displays the above event try to delete that event there and trigger a sync. Otherwise - if you don’t have real access to your data on a file basis - you can export the calendar into a file, delete that event manually in a text editor and import the data again ino a new calendar.
Hm, I guess the problem is the unescaped
; in the
X-APPLE-STRUCTURED-LOCATION line. I don’t think there is an easy way to work around that. Creating a new calendar without that event like suggested by @devvv4ever is probably the best solution.