how to host and export my availability ics calendar and sync it to other hosts like booking.com or homeaway? - import

Question

I am using my own selfmade php calendar. Every time I update the calendar, an ics file is created and placed in the "ical" directory on my server under the name "24633.ics". I can import the file into my Google calendar, but it fails on other booking websites. The file complies with the ics standards. What went wrong? What are the requirements for the calendar or server to import the file from booking websites?
Address: https://www.fewo4rent.de/ical/24633.ics
The calendar itself:
BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
PRODID:-//fewo4rent.de//DE
BEGIN:VEVENT
UID:1667560674-20221105-74919
DTSTAMP:20221104T121754Z
DTSTART;VALUE=DATE:20221105
DTEND;VALUE=DATE:20221112
SUMMARY:Belegt
END:VEVENT
BEGIN:VEVENT
UID:1667560674-20221127-59384
DTSTAMP:20221104T121754Z
DTSTART;VALUE=DATE:20221127
DTEND;VALUE=DATE:20221203
SUMMARY:Belegt
END:VEVENT
BEGIN:VEVENT
UID:1667560674-20221210-96485
DTSTAMP:20221104T121754Z
DTSTART;VALUE=DATE:20221210
DTEND;VALUE=DATE:20221224
SUMMARY:Belegt
END:VEVENT
BEGIN:VEVENT
UID:1667560674-20230715-55753
DTSTAMP:20221104T121754Z
DTSTART;VALUE=DATE:20230715
DTEND;VALUE=DATE:20230805
SUMMARY:Belegt
END:VEVENT
END:VCALENDAR
The headers:
header('Content-type: text/calendar charset=UTF-8');
header('Content-Disposition: attachment; filename='24633.ics')
or
header('Content-Disposition: inline; filename='24633.ics');
One website answers: "No reaction from the URL"