Assuming the identity user is MACHINENAME\ArcGISWeb and the database where the user accounts are stored in named "aspnetdb"1. The MACHINENAME\ArcGISWeb user needs to be added to the logins in the SQLServer with "aspnetdb" set as the default database.2. And in the user mapping for this user, allow aspnet_membership_FullAccess and aspnet_Roles_FullAccess.