hm what you describe happend to my several times even with FortiClient 6.4. It also happens with 7.0. Sometimes it works when one tries for a while.
IKE Log on my FGT shows the tunnel comes up completely (I see everything from p1 handshaking proposals over xauth and psk auth and even mode config. Tunnel gets enumerated and client gets IP and routing via mode config.
So VPN seems to be established just Forticlient for whatever reason doesn't get that and stays stuck at connecting until finally the vpn gets an idle timeout or similar.
It does not throw any errors in log.
Though if I use an alternative Client the tunnel comes up like a charm and works fine. So this seems definitely to be some forticlient issue.
I think about opening some TAC Ticket on this...
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