Calendar is not generated

There is a custom calendar of my client that generates a file for each apartment room “https://mydomain.com/wp-admin/admin-ajax.php?action=generate_ics_file&room=xxx”. It seems that it generates not .ics file, but .ics content https://ibb.co/hmXgK8c. When I add any URL of the client’s calendar to ICS calendar on my website, the page with ICS calendar crashes (infinity loop or something). Synchronization with booking.com works well, so the client wants that I fix this issue on my website, not on the client’s who has a custom calendar. Do you have any idea how to fix it?