Narrowed it down a bit.. After i Unchecked the Remote Login Optin in WordPress MU Domain Mapping, i suddenly get the same error as him:
http://wordpress.org/support/topic/error-msg-invalid-key-woocommerce-plug-in?replies=1
>Please enter your username or email address. You will receive a link to create a new password via email.
> Sorry, that key does not appear to be valid.
Wordpress still dosen't add a User in the DB :/
But the Registration of Accounts over the "My Account" page works without Problems, it's just on the Checkout page.