Solved that issue by myself. You have to add your Stripe payment methods to the session-array of payment_method_types in file BxPaymentProviderStripeV3.php. It contains only "card" payment method in the original file. Unfortunately, payment method types intended for one-time payment only, will not work with a subscription product or service you offer. It gave me the error message 'invalid_request_error; The payment method sofort cannot be used in subscription mode", for example ('sofort' is a common direct payment method in my country).