Andreas, indeed this a problem from Booking.com
This is their response to my inquiry:
"There currently is a bug out with our Calendar systems where our Partners can not Export Calendar or Sync accurately on other platforms.
We have our technical team currently working hard on trying to resolve this issue. When they have resolved they will send out a message to all of our partners stating so."
So all we can do is wait, unfortunately.
Instead of delisting the property i deactivated instant booking for now.