Hi @pasin,
The x.509 alternative name is an extension field to indicate other possible names or identities for the machine, for example if it has multiple names or if you want to be able to address the machine by name or by IP.
Regarding the issue you have, here is a link for you to review: http://forum.mikrotik.com/t/win10-ike-vpn-authentication-credentials-are-unacceptable/155863/1
Let me know whether that answers your question and solves the issue.