A Ghost Appointment

A probably found a bug which happened other times in the past but was never resolved because it's really difficult to reproduce: an appointment slot appears booked on the front even if there is no appointment scheduled for that slot. All configurations seems correct. That's why I call it the "ghost appointment".
If we act quickly you can now see the bug live in my web site.
I activated the support access. Please see
http://adventurerooms.it/perugia/en/booking-original-challenge/
Select 30th november (tomorrow) and check appointment at 20:30. It appears booked.
If you go in the admin panel you'll see there no appointment for that slot, and previous and subsequent appointments does not overlap.
Please consider I have the padding add-on activated.
I would be very grateful if you could examine the bug before tomorrow, as it's really very difficult to find another occasion to see it live.
Thanks a lot for your support.
Kind regards
Antonio