The host allowed me to reserve, but if I click reserve, it switches to Airbnb host page and writes that dates are not available.
I wrote a message to the host with requested dates. Host told me: ok. I have 24 hours to finalize. After some hours I come back and the apartment is already booked, but only in my dates, otherwise it's free.
If the listing is new, it takes some time to be fully accesable/bookable. It then can produce the message you received. If host sent you a special offer, you have 24 hours to accept. But dates are not reserved, so somebody else can book as long you do not accept. Or maybe the host blocked the dates manually. So you can only ask him/her why dates are not available (anymore)
It turned out the house was really booked. The owner claims somebody booked the house. But the house is free from next Monday untill the 20 December. I wanted to book from 20 to 29 Dec. More realistic I think is that I wanted to book the week around Christmas, the onwer is probably busy around that dates and he forgot to delete the dates from the calendar. I found another solution. Period.