Appointments+: time slots show busy when I am not

I have 2 working days, Friday and Suturday. Friday working hours are 9:00 - 17:00 and Suturday 9:00 - 15:00.
When I'm trying to book a time slot on the frontend and choosing Suturday, it shows that 15:00, 16:00 are busy time slots. But I don't have any appointments yet.