Error: Padding is invalid and cannot be removed. pls help

[Robert Smith]

I am getting the following error being caught in my application_error
method...
I have no code referencing the crypto classes. I have found nothing on
the net outside of people with crypto issues. Please Help.
~rob
Padding is invalid and cannot be removed.

at
System.Security.Cryptography.RijndaelManagedTransf orm.DecryptData(Byte[]
inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer,
Int32 outputOffset, PaddingMode paddingMode, Boolean fLast)
at
System.Security.Cryptography.RijndaelManagedTransf orm.TransformFinalBlock(Byte[]
inputBuffer, Int32 inputOffset, Int32 inputCount)
at System.Security.Cryptography.CryptoStream.FlushFin alBlock()
at
System.Web.Configuration.MachineKeySection.Encrypt OrDecryptData(Boolean
fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length,
Boolean useValidationSymAlgo)
at System.Web.UI.Page.DecryptString(String s)
at
System.Web.Handlers.AssemblyResourceLoader.System. Web.IHttpHandler.ProcessRequest(HttpContext
context)
at
System.Web.HttpApplication.CallHandlerExecutionSte p.System.Web.HttpApplication.IExecutionStep.Execut e()
at System.Web.HttpApplication.ExecuteStep(IExecutionS tep step,
Boolean& completedSynchronously)