System.Security.Principal.WindowsIdentity.get_AuthenticationType() throws exception

Can anyone tell me what the minimum user rights are to successfully call this method each and every time, under IIS 6.0 Windows Authentication with identity impersonation on?

I'm getting one of the following errors recorded in my event log, at least once every day:

System.Security.Principal.WindowsIdentity.get_AuthenticationType()
   at System.Web.HttpRequest.CalcDynamicServerVariable(DynamicServerVariable var)
   at System.Web.HttpServerVarsCollectionEntry.GetValue(HttpRequest request)
   at System.Web.HttpServerVarsCollection.GetServerVar(Object e)
   at System.Web.HttpServerVarsCollection.Get(Int32 index)
   at System.Web.HttpServerVarsCollection.GetValues(Int32 index)
   at System.Collections.Specialized.NameValueCollection.Add(NameValueCollection c)
   at System.Web.HttpRequest.FillInParamsCollection()
   at System.Web.HttpRequest.GetParams()
   at System.Web.HttpRequest.get_Params()
   at AjaxControlToolkit.ToolkitScriptManager.OutputCombinedScriptFile(HttpContext context)
   at AjaxControlToolkit.ToolkitScriptManager.OnInit(EventArgs e)
   at System.Web.UI.Control.InitRecursive(Control namingContainer)
   at System.Web.UI.Control.InitRecursive(Control namingContainer)
   at System.Web.UI.Control.InitRecursive(Control namingContainer)
   at System.Web.UI.Control.InitRecursive(Control namingContainer)
   at System.Web.UI.Control.InitRecursive(Control namingContainer)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I don't believe it's the AJAX Toolkit. How can a user interface library have anything to do with security? I believe the error is occurring when it accesses Request.Params and authenticationtype is being populated, but if thats the case - why is the AJAX toolkit always being reported as the culprit?

Cheers,

Scott