This error may occur when you are trying to generate a SSL certificate for internal domain names like internalname.local. If you choose email validation prefix e.g. "administrator@" this will produce an invalid email address "administrator@interne-adresse.local" and thus resulting in this error.
We cannot create SSL certificates for internal domain names like .local or .internal, for further details please read https://www.interssl.com/de/knowledgebase.php?action=displayarticle&id=162
