If you get an error message saying sickness or leave already booked in this date range when trying to close a sickness, this is because the employee has a leave booked on that date.
You must close the sickness before the holiday date and then open a new sickness that continues after the holiday, or you must delete the holiday in that date range.
📌Note: If you are unable to book leave and the same error message comes up, you must close the sickness first.