Padding is invalid and cannot be removed

Hi there,
I've noticed a number of events being logged on our Windows server, coming from an Asp.net application. I wouldn't know where to begin troubleshooting this problem, but the basics are this:
- Windows Server 2003 Standard Edition (NLB)
- Asp.net 3.5 App
- Oracle 10g DB
Event ID: 1309
Event code: 3005
Event message: An unhandled exception has occurred.
Exception information:
    Exception type: CryptographicException
    Exception message: Padding is invalid and cannot be removed.
The second last line here is why I posted in the security forum.
The page that it's coming from is webresource.axd
Stack trace:    at System.Security.Cryptography.RijndaelManagedTransform.DecryptData(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer, Int32 outputOffset, PaddingMode paddingMode, Boolean fLast)
   at S ...