As the SW works fine with many other VPNs (to other routers with dynamic or static public IPs) I think the problem is located in the Cisco ACLs (I've also been able to establish a VPN between these two devices... but in Main Mode with static IPs), because trying to establish connection is up to one or two code lines --though it fails at Phase 2--.
I'll put some code to explain the situation:
crypto isakmp policy 20
crypto isakmp peer address XXX.XXX.XXX.XXX
set aggressive-mode password XXX
set aggressive-mode client-endpoint user-fqdn XXX@XXX.XX