Padding is invalid and cannot be removed.

Greetings,

I have all application errors sent to my email. I keep getting this:
mysite.com/webresource.axd?d=yacsx7hz0irzn_i7ifr3morrek9u6srdkqxsjvpn3mw1&t=633598204507656250
Padding is invalid and cannot be removed.

[Exception Info]

    Stack Trace:   at System.Security.Cryptography.RijndaelManagedTransform.DecryptData(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer, Int32 outputOffset, PaddingMode paddingMode, Boolean fLast)
   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 ...