Would anybody have an idea ?

Uhm... sorry, maybe I can't understand exactly what you mean due to the fact that english is not my primary language but..

The customers should access the system through the customer interface (customer.pl): so they usually don't have need a role, since they can only  "read only" and "create" by definition.

They can only belongs to some group, just in case you have more than one queue and you need to hide some of those queues from customer's eyes.