Site-to-Site VPN IPsec phase 1 error

[Lester_Tuvera_Marcos]

Hi Sir/Ma'am,

We've created a new site-to-site VPN and got the alert message below. Does anyone know the meaning of the error message? I'm using Fortigate 1000C and my peer is Sophos SG320. Have double checked the phase 1 proposal parameters (authentication, encryption, etc.) and everything seems ok.

Message meets Alert condition date=2017-06-28 time=14:23:40 devname=FG300B3908605958 devid=FGT1KC3914800273 logid=0101037124 type=event subtype=vpn level=error msg="IPsec phase 1 error" action=negotiate remip=146.88.74.50 locip=115.85.17.114 remport=500 locport=500 outintf="port12" cookies="69b6e894c1236b86/c4ab30e19afc73f9" user="N/A" group="N/A" xauthuser="N/A" xauthgroup="N/A" vpntunnel="BancNet_Instapay_VPN" status=negotiate_error error_reason="peer notification" peer_notif="INVALID-MESSAGE-ID"   Thanks.