Dynamics CRM 2011 Outlook Client Invalid OrganizationId

I suddenly found myself unable to configure any Dynamics CRM 2011 Outlook client, receiving the following error:

>Failed to refresh entry. Exception Microsoft.Crm.Application.SMWrappers.InvalidOrganizationIdException: Invalid OrganizationId "xxxxx566-5xxx-e1xx-adxx-xxxx56af0xxx".
   at Microsoft.Crm.Application.SMWrappers.ClientOrganizationContextFactory.Get(Guid organizationId)
   at Microsoft.Crm.Application.SMWrappers.ClientOrganizationContextFactory.Microsoft.Crm.IOrganizationContextFactory.GetOrganizationContext(Guid organizationId)
   at Microsoft.Crm.Caching.CrmMultiOrgCache`2.RefreshClientEntryCallback(String key)
   at Microsoft.Crm.ClientCrmCache`1.RefreshEntryWaitCallback(Object keyString)


It seems this happened after enabling SPN's to work with the CRM 2011 FetchXml reports. The resolution is to enable IIS to authenticate using the AppPool Account credentials.

Resolution
In IIS highlight the CRM website and open the Configuration Editor

Navigate to:  
system.webServer > security > authentication > windowsAuthentication

Set useAppPoolCredentials to True