It looks like the move of the peer-group definition for IPv6 address family BGP neighbors is not migrated during the upgrade from 1.3.5 to 1.4-RC1. Here is a screenshot of the KVM console :
I assume the migration scripts just need to move it from protocols bgp <ASN> neighbor <NEIGH> address-family ipv6-unicast peer-group to protocols bgp neighbor <NEIGH> peer-group