This isn't a CS bug. ASP.NET 2.0 requires additional changes.
Set a MachineKey in your web.config (not only in the AppSettings).
Also, in ASP.NET 2.0 the RolesProvider Timeout is honored (in the past it was ignored on 1.1). Open your web.config and edit the RolesProvider section for the Timeout.
Again, you'll also need to properly setup your <MachineKey />:
In section of the web.config change the Timeout from 90 to 525600 (1 year).