Invalid value for 'encryptedTicket' parameter

[Aamir79]

I am using basic forms authentication in my web application and it is working
fine on my Dev machine. However when I stress test the application using
Microsoft ACT,
And increase the number of concurrent users more than 3, I get following
error:

Exception Type: System.ArgumentException
Message: Invalid value for 'encryptedTicket' parameter.
ParamName: NULL
TargetSite: System.Web.Security.FormsAuthenticationTicket
Decrypt(System.String)
HelpLink: NULL
Source: System.Web

StackTrace Information
*********************************************
at System.Web.Security.FormsAuthentication.Decrypt(St ring encryptedTicket)
at Agent.Global.Application_AuthenticateRequest(Objec t sender, EventArgs
e) in c:\inetpub\wwwroot\Sia.Agent\Global.asax.vb:line 131

The line causing error is :
Dim ticket As FormsAuthenticationTicket =
FormsAuthentication.Decrypt(Context.Request.Cookie s(roleCookie).Value)
Up to 3 browser connections it works fine without any error. Same code was
working perfectly a couple of days back with up to 100 concurrent connections.

It seems the error is when the Cookie is being decrypted but I don’t
understand why the error occurs only when the number of concurrent users
increases.

I’m using ASP.net framework 1.1, IIS 6.0, Windows server 2003 enterprise
edition with service pack 1.

Please Advice!