Dialup VPN Log on server: Cookie already in use

Hi,

I have a FGT61E with a USB 4G dongle with a static public IP acting as a dialup VPN server, and a FWF61E with a USB 4G dongle behind a CG NAT acting as the dialup client.

I am trying to follow this article, but it's not very good IMO. http://help.fortinet.com/fos60hlp/60/Content/FortiOS/fortigate-ipsecvpn/FGT_Dialup_Client_Config/FG_dialup-client.htm?Highlight=vpn%20dialup

I cannot get the tunnel up, and when I look on the debug on the CLI, I see an entry that goes something like: 

ike 0:<vpn name>:91: drop aggressive from <dialup ISP IP>:500-><dongle USB IP> 30 cookie <cookie sig?>, cookie <cookie sig?> already in use

I cannot find anything on google about what this cookie is all about... please help.

Thank you