I have a standard Shrepoint 2007 setup which I've published through an ISA
2006 server with an https listener which uses Windows authentication.  The
client to the ISA server traffic is all https and between the ISA server and
the Sharepoint server its clear http.  The Sharepoint databases are on a
separate SQL server.  Access to most of the site functionality is fine but
when I go into a start workflow form and enter a name and press the validate
name button I get an Infopath Form warning:

Warning                                                   Powered by:
Infopath Forms Services

The form cannot be submitted to the Web server either because your computer
is offline or because the host server is currently unavailable. If this
problem persists, contact your network administrator.

This stops the form submitting.

My computer is not offline and the host server is available - any help much
appreciated.