Object reference not set to an instance of an object at Microsoft.Exchange.Transporter.InternetProxy.GetWebProxy(IReportProgress report)

I'm trying to call the Set-MSOnlineUserPassword cmdlet (part of the Microsoft Online Services Migration Tools) to reset a BPOS user password from a web application running under Network Service identity on Windows Server 2003 SP2. The following exception gets thrown:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Exchange.Transporter.InternetProxy.GetWebProxy(IReportProgress report)
   at Microsoft.Exchange.Transporter.Provider.MicrosoftOnline.MicrosoftOnlineSessionContext.GetAWSProxy(IReportProgress report)
   at Microsoft.Exchange.Transporter.Provider.MicrosoftOnline.MicrosoftOnlineSessionContext..ctor(ScenarioStep step, ScenarioConfig config)
   at Microsoft.Exchange.Transporter.Provider.MicrosoftOnline.MicrosoftOnlineUserActivator.OpenTargetSession(ScenarioStep step, ScenarioConfig config)
   at Microsoft.Exchange.Transporter.ScenarioDirector.PrepareToExecute()
   at Microsoft.Exchange.Transporter.ScenarioDirector.ConfigureSession(ScenarioConfig config)
   at Microsoft.Exchange.Transporter.PSI.BaseTask.PrepScenario()
   at Microsoft.Exchange.Transporter.PSI.BaseTask.BeginProcessing()
   at Microsoft.Exchange.Transporter.PSI.SetXsMicrosoftOnlineUserPassword.BeginProcessing()
   at System.Management.Automation.Cmdlet.DoBeginProcessing()
   at System.Management.Automation.CommandProce开发者_如何学JAVAssorBase.DoBegin()

The same code runs fine if I change the app pool identity account to use Local System so it appears to be some kind of permission issue but there are no security errors in the Event Log. Does anyone know what could be the specific permission that the Network Service account is missing?

The problem was resolved by using a domain account instead of the local Network Service account to run the app pool.