Order status doesn't change after successful Paypal payments

-- url of the page with the problem -- : nosch.de
-- HikaShop version -- : 5.0.3
-- Joomla version -- : 4.4.3
-- PHP version -- : 8.1.27
-- Browser(s) name and version -- : Chrome 122.0.6261.69

I am using the Paypal Checkout plugin in Sandbox mode.
It works and I can pay orders and I'm forwarded to the succes-page.
In my paypal sendbox event I see the payment is completed. But the state doesn't change in Hika-shop, it remains "created".

I used the personal sandbox account for the payment that belongs to the paypal sandbox account.

These entries I could find in the Hikashop log:

31S34550BB561251E
02.28.24 14:50:22

{"error":"invalid_client","error_description":"Client Authentication failed"}
02.28.24 14:50:22