Access is denied in WindowsIdentity.GetCurrent

[Robert Phillips]

I am suddenly getting this on a couple of Windows XP (.NET 1.1) machines in
my environment. The machines haven't change, and my code hasn't changed.

ExceptionType="System.ApplicationException"
Message="Access is denied."
TargetSite="IntPtr _GetCurrentToken()"
Source="mscorlib"
at System.Security.Principal.WindowsIdentity._GetCurrentToken()
at System.Security.Principal.WindowsIdentity.GetCurrent()
at System.AppDomain.GetThreadPrincipal()
at System.Threading.Thread.get_CurrentPrincipal()
at my code.....

my code is:

AppDomain.CurrentDomain.SetPrincipalPolicy(PrincipalPolicy.WindowsPrincipal);

//exception happens here
string user = System.Threading.Thread.CurrentPrincipal.Identity.Name;

I originally thought it was a CAS issue, but SetPrincipalPolicy works which
means that the ControlPrincipal SecurityPermission is not the issue.
Reflector shows the _GetCurrentToken() method as

[MethodImpl(MethodImplOptions.InternalCall)]
private static extern IntPtr _GetCurrentToken();

which i believe means it is an internal call into the CLR.

Does anyone have any thoughts on why this would be happening?