[eluser]WanWizard[/eluser]
You also get this error when the user_data field is defined as 'NOT NULL' (which is how it is defined in the manual).
This is wrong, because when the session is created, the user_data field is not included in the INSERT query.
I think you have to report this as an issue. Either the manual has to be modified, or the user_data field has to be properly initialized when the session record is created.