The "Invalid Cookie" message was received during the IKE exchange. It means that one of the VPN endpoints (Client or peer) is using a tunnel which is no more in use.
Note: this warning may also occur when closing the VPN Tunnel. It is not a blocking problem.
Console message example:
20090429 115933 Default message_recv: invalid cookie(s) 5918ca0c2634288f 7364e3e486e49105
20090429 115933 Default dropped message from 195.100.205.114 port 500 due to notification type INVALID_COOKIE
20090429 115933 Default SEND Informational [NOTIFY] with INVALID_COOKIE error | 
