VPN error 781: cannot connect…

At my office, we are running Windows 2000 server with service pack 3. It is set up for remote acces and all users that need be, have dial in permissions. We have 15 to 20 working vpn connections per day and only one that cannot connect. This connection is configured correctly, but the only problem is that they are attempting to connect from behind a firewall and continue to recieve the error 781 message: attempt failed due to no valid certificate. Is there any thing i can do here at our end, oris this issue due to the firewall she is behind?? And what would be the best solution in your opinion??? I would appreciate any and all feedback.