If you are connecting to a KX III standalone device and support for AKC download server certificate validation is enabled, the valid IPv6 format to generate the certificate is either:
CN =[fd07:02fa:6cff:2500:020d:5dff:fe00:01c0]
when there is a leading 0 or
CN =[fd07:02fa:6cff:2500:020d:5dff:0000:01c0]
when there is no zero compression