PayPal has unexpectedly returned a blank address

[mibo21]

Hi,
I have a Zen Shop (1.3.9) up & running with activated PayPal Express module for payment.

If a customer is not registering manually, but directly bypassing the whole registration part by using the paypal express method, everything works fine. Cashflow works, Account is created, Email to the customer... all good!

But if an already registered customer chooses 'PayPal' as payment method at the end of the order process it does not work. Paypal login page will load as it should, but after entering the customer paypal credentials Zen-Cart returns the following error message:

PROBLEM: We're sorry. PayPal has unexpectedly returned a blank address. <br />In order to complete your purchase, please provide your address by clicking the &quot;Sign Up&quot; button below to create an account in our store. Then you may select PayPal again when you continue with checkout. We apologize for the inconvenience. If you have any trouble with checkout, please click the Contact Us link to explain the details to us so we can help you with your purchase and prevent the problem in the future. Thanks.'

Any idea where the root cause is located? I already reviewed some threads and learned that there were some issues with Zen 1.3.8 and the blank adress, but those should be fixed in Zen 1.3.9.

Thanks for any idea, where I can approach to do some more bugfixing / testing. The IPN configuration looks good to me as long as the express registration & payment works, or am I mistaking?

Looking forward to some answers of the specialists...

Best regards,
Michael