asterisk users - Jul 2010 - Fax T.38 passthrough failing after upgrade

Hello all!

After a recent upgrade of Asterisk 1.4.18 to 1.4.33.1, everything
looks ok, except when fax T.38 calls are being passed through the
server, the fax transmission fails, giving a "train failure" message
when invoking "fax show stats" in the fax server (Asterisk 1.6.0.26).
Rxfax and Txfax are not installed on the passthrough server.  This was
working fine before the upgrade.

Here is a quick topography diagram:

T.38FaxATA -> Internet -> Asterisk 1.4.33.1 passthrough -> Asterisk
1.60.26 Fax Server

What is causing this error and how do you think it can be corrected?

Thanks!
Elliot