[otrs] Problem to edit customers data from external backend

Hello, I have a question about the use of an external backend. I manage to link my OTRS system with my existing customer backend. So when I create a new customer in "admin -> Customer Users" it is correctly created in my DB. When I search for a customer, they are also correctly found in my DB. My problem is when I want to load/edit the data of an existing customer (Edit (Change) Panel). The fields' values are not retrieved and are blank. Has anybody already faced this problem or does know how to solve this? Thank you in advance Best regards Fabien