It seems that in RouterOS v6.48.6 (Long-term) it is not possible to use Site-to-Site L2TP-VPN with IPv6 only (we tried on RB2011).
Server Setup is possible, but if one tries to configure the Client, one runs into the problem that the “Connect To” field only accepts IPv4 addresses (see also L2TP.PNG).

Is there any possibility to do it anyways?
Thanks!