No login possible via LDAP plugin with Contao

Question

For our new intranet we want to use Contao. So that we do not want to create each employee by hand. One of the requirements is that LDAP users can log in to the frontend. In this regard there is a plugin from con4gis ldap. So far everything works and the employees are also imported and displayed. However, the login does not work. There is always the error message "invalid Passoword". Does anyone have an idea what the problem could be.

Here are the configurations. I have replaced sensitive data with ***.

security.yml

security:
    providers:
        con4gis_ldap:
            ldap:
                service: Symfony\Component\Ldap\Ldap
                base_dn: 'dc=intern,dc=***,dc=de'
                search_dn: 'cn=service_intranet,ou=Service,ou=IT,ou=Mitarbeiter,dc=intern,dc=***,dc=de'
                search_password: '***'
                default_roles: ROLE_USER
                uid_key: sAMAccountName
                filter: '(objectCategory=person)'

        chain_provider_backend:
            chain:
                provider: [contao.security.backend_user_provider, con4gis_ldap]

        chain_provider_frontend:
            chain:
                provider: [contao.security.frontend_user_provider, con4gis_ldap]        

    firewalls:  
        contao_backend:
            provider: chain_provider_backend
            form_login_ldap:
                service: Symfony\Component\Ldap\Ldap
                dn_string: 'uid={username},ou=Service,ou=IT,ou=Mitarbeiter,dc=intern,dc=***,dc=de'

        contao_frontend:
            provider: chain_provider_frontend     
            form_login_ldap:
                service: Symfony\Component\Ldap\Ldap
                dn_string: 'uid={username},ou=Service,ou=IT,ou=Mitarbeiter,dc=intern,dc=***,dc=de'

And here is a picture of the settings in the backend