Issues with ASP.NET app and hyperthreading when stress testing?

[GeorgeB]

HI,
I created an ASP.NET application which runs on win 2003 server and IIS6 with
hyperthreading enabled.
I keep getting the following error:

Path: /Login.aspx.
Source: System.Web
at System.Web.UI.Page.LoadPageStateFromPersistenceMed ium()
at System.Web.UI.Page.LoadPageViewState()
at System.Web.UI.Page.ProcessRequestMain()

Nested Exception

Exception: System.Web.HttpException
Message: Authentication of viewstate failed. 1) If this is a cluster, edit
<machineKey> configuration so all servers use the same validationKey and
validation algorithm. AutoGenerate cannot be used in a cluster. 2)
Viewstate can only be posted back to the same page. 3) The viewstate for
this page might be corrupted.
Source: System.Web
at System.Web.UI.LosFormatter.Deserialize(String input)
at System.Web.UI.Page.LoadPageStateFromPersistenceMed ium()

Nested Exception

Exception: System.Web.HttpException
Message: Unable to validate data.
Source: System.Web
at System.Web.Configuration.MachineKey.GetDecodedData (Byte[] buf, Byte[]
modifier, Int32 start, Int32 length, Int32& dataLength)
at System.Web.UI.LosFormatter.Deserialize(String input)

I am not running it on a cluster, but just in case I change the machinekey
section in machine.config to a hardcoded key. Upon login in the app creates
a Thread.CurrentPrincipal and then does a Response.redirect to another aspx,
it seems like it's never actually being redirected.
I enabled webfarm in the Application Pool in IIS (I assume that if I have 2
processors which are hyperthreaded resulting in 4 processors I should set the
# worker process to 4).

After doing the above, I still get this problem. It should be noted that I
can only reproduce this on that hyperthreaded server (I have no other
hyperthreaded server and disabling hyperthreading is not a solution), when I
try it on another server that does not use hyperthreading everything is fine.
Also this can only be reproduced when I run stress test on the server (I use
ACT). When I manually login I do not get errors.

Has anyone else experienced this? Or something remotely simlar ;) ?

TIA,
George