We have 2 servers (server-A on internal network, and server-B on the DMZ).

Server-A connects via sftp to servers on internet using server-B as proxy. Everything was working normally until few days ago. On the server-A logs, we started noticing this exception:
com.maverick.ssh.SshException: Unexpected terminaton: Conduit is closed

On the server-B (proxy on dmz) logs, we saw:
[2010-08-31 20:30:07,063] ERROR Per[-]: read error: java.io.IOException: Connection reset by peer
[2010-08-31 20:30:07,063] INFO Per[-]: close: CloseCode.CONNECTION_RESET

We checked the firewall, network and everything else that we could think of.. None of these layers had any changes recently.. Finally we noticed the system time on server-B was off by 6 minutes compared to server-A. When we synch'd the time on server-B with server-A, sftp connection started working. We still cannot believe that we have fixed the issue, is timestamp that critical for a successful sftp connection? Are we missing anything else? Please let me know.