ssh refuses connections

[Jason Majors <jason@whizzird.net>]

I have four machines: a firewall, an internal server, and two workstations.
When I try an SSH connection from the firewall to the server, or from outside
the firewall using its port forwarding to go to the server, I get the error:
ssh_exchange_identification: Connection closed by remote host

I can connect just fine from the two workstations to the server, the server to
the workstations, or any of the internal boxes to the firewall.
The only changes that I've made since it last worked were the installation
of NIS on the server (which has since been removed), and some NFS changes.

Any help would be appreciated.
Thanks,
Jason