Length of the data to decrypt is invalid error

I occasionally get the following error, and I have no idea where it is coming from. It seems to be something internal to asp.net.
 

Error Message: Length of the data to decrypt is invalid.

Stack Trace:    at System.Security.Cryptography.RijndaelManagedTransform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)


  at System.Security.Cryptography.CryptoStream.FlushFinalBlock()

  at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Boolean useValidationSymAlgo)

  at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString)  
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