Opt-in causes WooCommerce Checkout fatal error 400 invalid parameter extensions

Dear all, we enjoy MailPoet, but we cannot use it on the new WooCommerce block checkout.

After activating MailPoet: WooCommerce: Opt-in on checkout page (“Opt-in auf Bezahlseite”), customers see payment options in checkout, but when trying to complete checkout (everything is correctly filled in, including valid credit card payment data) by clicking on the complete checkout button (“kostenpflichtig bestellen”), the checkout process fails with a fatal 400 error due to invalid parameter: extensions.

The customers see a red circle with white exclamation mark icon and an error message (localized in German: “extensions > mailpoet ist nicht vom Typ object.” (extensions > mailpoet is not of type object). The error message is displayed as a dismissable notice (.woocommerce-error components-notice.is-dismissible .components-notice__content) but the error prevents the customers to complete the checkout.

After deactivating the checkout opt-in feature, users can complete the checkout successfully.

WordPress 6.1.1
PHP 7.4.33
MailPoet 4.0.1
WooCommerce 7.1.0
WooCommerce Blocks 9.0.0
WooCommerce Payments 5.0.3
WooCommerce Shipping & Tax 2.0.0
Germanized für WooCommerce 3.11.1
Germanized für WooCommerce Pro 3.6.2

Request Status:
/wp-json/wc/store/v1/checkout?_locale=user
Failed to load resource: the server responded with a status of 400 ()

Response Details:

{"code":"rest_invalid_param","message":"Ung\u00fcltige(r) Parameter: extensions","data":{"status":400,"params":{"extensions":"extensions > mailpoet ist nicht vom Typ object."},"details":{"extensions":{"code":"rest_invalid_type","message":"extensions > mailpoet ist nicht vom Typ object.","data":{"param":"extensions > mailpoet"}}}}}

Request Payload:

{"billing_address":{"first_name":"John","last_name":"Doe","company":"","address_1":"Teststr. 123","address_2":"II (Vorderhaus)","city":"Berlin","state":"","postcode":"12043","country":"DE","email":"[email protected]","phone":""},"customer_note":"Testbestellung wegen Zahlungsarten vs. Mailpoet opt-in","create_account":false,"payment_method":"woocommerce_payments","payment_data":[{"key":"paymentMethod","value":"woocommerce_payments"},{"key":"wc_payment_intent_id","value":"pi_3M8iYfFmV1RD0GLh14yzYImX"},{"key":"wcpay_selected_upe_payment_type","value":"card"},{"key":"wc-woocommerce_payments-new-payment-method","value":false}],"extensions":{},"shipping_address":{"first_name":"John","last_name":"Doe","company":"","address_1":"Teststr. 123","address_2":"II (Vorderhaus)","city":"Berlin","state":"","postcode":"12043","country":"DE","phone":""}}