I'm bumping this. I've managed to get a little bit forward. I got the tunnel up using IKEv1 but on the Juniper side of the configuration I had an interface linked to the inside of the tunnel, which makes it a "route based VPN". I can't find anywhere in the MAS to give the inside of an IPSEC tunnel an IP address, is there such setting?
If not, we need to go for "policy based VPN" but as soon as I remove that interface linked to the inside on the juniper side of the tunnel, the tunnel goes down.
Any advise would be helpful.