[TYPO3-commerce] checkout email error message

[the machine_whisperer]

Hi there!

I have succesfully created a commerce installation, finally.
(More on that later)

What still bugs me is the fact that I still receive an error message
after the order has been finalized:

The transaction was completed successfully
Your email address is not valid. Please try again.

But both emails to the customer as well as to the admin are delivered 
succesfully.

Has anybody here had the same problems?

Kind regards,

Markus