Yes, @Karl100, it happened here last week, after years of it never happening. In my case it was a system-blocked date for prep. It was a reschedule, and I did end up approving it, but it was highly inconvenient to do a same-day turnaround that day. It's why I have ticked the prep-time box. I did contact Airbnb via twitter to see what I could do to ensure it never happens again.
Nothing really, except complain.
They explained that the problem came because the guest wanted two days and entered that, could not get them, so shortened their booking to one day and the blocked prep day magically (my word, not CC's) opened for them. Guest was thrilled. The developers are "looking into it".
Since yours is an actual booked date, you'll have to contact Airbnb, too. I suspect it is some sort of loophole like mine was. Guest wanted a date, couldn't get it, changed something, and voila!
But will you let us know?
