Hier die Antwort der Entwickler ...
Hi,
Thanks you for contact us, The cause is PayPal refuses to load the block "payment-selection"
It is block is obligatory for good work of the module (it allows to choose the type of payment: cart, PayPal account etc.).
Normally it happens because the PayPal account is not associated with Germany (the account must have linked with Germany).
Thank you, to check if his account is associated with Germany
Ich habe jetzt eine neue Sandbox REST-API erstellt mit Sandbox developer Account (DE) und danach eine neue Live REST-API ...jetzt funktioniert die Weiterleitung bei uns ...