unable to serialize the session state. .. .. . .bla bla bla

Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode
I am getting the above error message, what could be the problem? i think it has to do something with ...