Can not you find the appointment in the list of appointments via the customer base?
Do you have an appointment in the agenda, but do not you see it when you look up the customer in the customer base? (under the heading of appointments).

This is possible, because when a customer is removed from the customer base, the corresponding agreements are not removed from the agenda for safety reasons. As a user you can recreate the same customer after deleting the customer. At that moment you will not find any appointments in his / her appointment list with the last created customer. These agreements are still on the customer's chip of the removed customer. Once a customer has been removed, you can not look it up or call it back.

In order to avoid this confusion, it is best to note the appointments from the customer file when deleting a customer from the customer base and also delete them immediately from the agenda. When the new customer card is created afterwards, you can re-enter the appointments in the agenda on the new customer.
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