Replace the actual route configuration:
Before
synolia_gdpr:
resource: "@SynoliaSyliusGDPRPlugin/Resources/config/routes/admin/customer.yaml"
prefix: '/%sylius_admin.path_name%'
After
synolia_gdpr:
resource: "@SynoliaSyliusGDPRPlugin/Resources/config/routes.yaml"
prefix: '/%sylius_admin.path_name%'