Product Vendor Extension Paypouts: Unpaid Status not changed

Hi there,

I am testing woocommerce + products vendors extension commission pay with Paypal Payouts in Sandbox mode:
1. Paypal Sandbox Business Account has been activated for payouts.
2. A new REST API App has been created for this account
3. Creditials of the REST Api App have been put into Admin backend > woocommerce > setup > products > vendors form fields.
4. A payment-Test has been started from Admin backend > Commissions > Pay Commission (for a checked order having UNPAID comission status)
5. The Sandbox API Call History at developer.paypal.com says: Http-Status was successful for the endpoint /v1/payments/payout.
6. Paypal Sandbox Vendor Test account has received the commision payment!

PROBLEM:
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However the the commsion payment status at Admin backend > commisions > single order record DOES NOT has been cjanged. It still remains UNPAID!

Obviously the paypal sandbox REST API Response was not received/processed correctly.

My QUESTION is:
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How I can debugg the API Response processing in order to localize the error. May be the API Response is not the right one?

Thanks for help
Frank