Unable to serialize the session state

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.

Answer:

Make everything serialisable!

Simply declare the objects / types <Serialisable()> or [Serialisable()] if you’re C#