On older versions of OpenVPN, where IPv6 transport through the VPN tunnel was not supported, but the VPN client did have the IPv6 protocol enabled on the VPN tunnel adapter, and was attempting to send IPv6 traffic through the OpenVPN tunnel, this error could show up. If you do not need IPv6 you can either ignore this message safely, or disable the IPv6 protocol on the virtual network adapter. Alternatively you can update your software to a more recent version, server and client side, to resolve this problem.