Same for me.
following the instructions in this thread I imported the booking.com iCal into a Google Calendar and then imported in airbnb the iCal of that Google Calendar and this iCal is then imported in airbnb without error.
But still I am not sure if the sync is working: in airbnb calendar I indeed see the bookings (coming from booking.com), but all these bookings were created before the issue appeared yesterday, but I don't know if the booking.com iCal feed actually exposes the new bookings that I will receive in the future, because I didn't recorded yet any new bookings on booking.com.
What is making me worry is that if now I block a date on booking.com calendar the bookings.com iCal feed doesn't include the blocked date.
I also tested the booking.com iCal feed with an iCal validator and indeed it says that the format is not valid because it is missing the DTSTAMP property.
Indeed if I compare (text diff) the booking.com iCal feed and the google calendar iCal feed the google one hase the DTSTAMP property whereas the booking.com not.
Still is not clear to me if the problem is on booking.com side because they don't expose a valid iCal format or if airbnb changed is importer accepting only iCal with a valid syntax.
Hope someone will fix this soon
Thank you all for sharing the info