delete default category of appointment

Hello,

I have to do some Fine-Tuning for the Activities in Cloud for Customer (C4C). I want to change the categories which are assigned to the appointments. At first I create some new categories in the Fine-Tune and now I am in the setting "Assignment of Categories".

I could delete all the standard categories which were assigned to the appointment exept the categorie "0001 - Customer Visit". When I want to delete this categorie I get an error message that the category is in use - see screenshot below. I checked all created appointments but this category isn't assigned to one of it so I don't understand where this category is used. Do you have any idea where this category could also be used? Or why the deletion get blocked by the system?

Thanks,

Best Regards Jasmin