IPSec VPN Firewall problem

Hello,

I've got a Win XP laptop sitting behind a RH 2.2.16-22 (ipchains)
firewall that's been patched to support VPN masquerading. I already
have 2 other VPN clients working this way, but I just got a new one that
I need to connect to. I can successfully make the VPN connection, but
when I attempt to access the remote network through the tunnel, nothing
works...not even ping. I see the following in the system log:

Feb 2 01:22:17 <myfirewallname> kernel: ip_masq_out_get_isakmp(): zero
icookie, pkt fm <laptop ip> discarded

It looks like packets from that client are being dropped summarily
whenever that particular VPN is connected before even being sent to the
remote network, and I have no idea why. I can't seem to find any
information regarding this error anywhere.

Like I said, I've used two other VPN clients behind this firewall for
over a year, so I know the VPN masquerading is working properly. I also
know that the problem is on the firewall end because I've managed to use
the VPN client in question on the same laptop outside of this LAN. I
was hoping someone might be able to give me inkling of what causes the
zero icookie error.

Thanks in advance!

Jason Rust