Accessing DB Via Application Server; Session Has Expired Error

Hello.  I've created an Application Server and Web Designer Portal on separate servers in a lab.  Both are configured on the site for Windows Authentication and I can access either via a client using SSO (if both are configured to connect directly to the DB).  If the Portal is configured to connect to the DB via the Application server however, the user gets a "user / password" form and then the error above (Session has expired, log on again).  Another client with the admin tools installed can use them fine via the Application server.

Any suggestions / or guides to configuring the Application server / Portal pair please?

Thanks for any help,