This same behavior occurs when you use StateServer session state mode, but you also receive the following run-time error:
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.Runtime.Serialization.SerializationException: The type <namespace.classname> in Assembly <AssemblyName>, Version=1.0.676.30211, Culture=neutral, PublicKeyToken=null is not marked as serializable.
Exception Details: System.Runtime.Serialization.SerializationException: The type <namespace.classname> in Assembly <AssemblyName>, Version=1.0.676.30211, Culture=neutral, PublicKeyToken=null is not marked as serializable.