Function of Pay with Amazon Plugin

-- HikaShop version -- : 4.2.2
-- Joomla version -- : 3.9.1
-- PHP version -- : 7.2.24
-- Browser(s) name and version -- : every

Hi Team,

we're trying to install the "Pay with Amazon" Plugin.
After install and setup with merchant id and the other requested keys, we_re checking the functionality with a testuser, defined in amazon seller central.
We've also installed the java origin and the return url is: shop.agentur-praxis-marketing.de/index.p...ayment=paywithamazon

What's the fact?
During checkout we've got the pop-up for login with amazon in sandbox mode (also the button on page), after the first login with the testuser the page returns to a blank page. In this stage you're no logged in in HikaShop and nothing happens. There's no further payment status, but the order is still generated without any customer- or delivery informations from amazon.

If you try to repeat these steps, it isn't possible, because the amazon login page doesn't appear again.

Is anybody here to help us with a hint, for finding the right solving?
Is there another amazon plugin available?

Every hint will be highly appreciate.