ASP.NET 3.5 error on new Vista? Unable to serialize the session state.

I have had a VS 2008R2/ASP.NET 3.5 website, which has run successfully on my old xp WORKSTATION.
Just migrated the website to a new Vista workstation.  Testing the web site showed the following error message.

What does "Unable to serialize the session state" mean?   How to fix the error? 
TIA,
Jeffrey
Server Error in '/my' Application. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session ...