How to avoid this error: Session state has created a session id, but cannot save...

In our VB/ASP web application, we have a sub-app (separate application in Website) that runs as a .net 1.1 web application with many aspx pages.  We also have another sub-app that runs as a .net 2.0 web application, but it uses a separate application pool.  This normally works fine, but on an upgrade to the web application I'm getting this error when I try to start up the 'eService' (.net 1.1 application):
"Session state has created a session id, but cannot save it because the response was already flushed by the application".  Here is the remainder of the detail:

[HttpException (0x80004005): Session state has created a session id, but cannot save it because the response was already flushed by the application.]
   System.Web.SessionState.SessionIDManager.SaveSessionID(HttpContext context, String id, Boolean& redirected, Boolean& cookieAdded) +621
   System.Web.SessionState.SessionStateModule.CreateSessionId() +52
   System.Web.SessionState.SessionStateModule.DelayedGetSessionId() +65
   System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs) +592
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +138
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +65

I don't know what happened.  Anybody recognize this symptom and can provide a solution?