CryptographicException: Padding is invalid and cannot be removed

I get visitors (or bots?) visiting my site and sometimes the following error occurs:

System.Security.Cryptography.CryptographicException: Padding is invalid and cannot be removed.

Stack:
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.Page.DecryptString(String s) at System.Web.Handlers.AssemblyResourceLoader.System.Web.IHttpHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)


Method:
Int32 DecryptData(Byte[], Int32, Int32, Byte[] ByRef, Int32, System.Security.Cryptography.PaddingMode, Boolean)


Source:
mscorlib

This error occurs when using a URL such as http://www.rolinka.com/WebResource.axd?d=3us2Y5XaY9P0iXpZPfPeZQ2&t=633253311525842705

This error appears to occur straight away, even before any code in the Session_Start event in Global.asax is executed.


Does anyone have any idea what the cause of the problem is, and how to fix it?

Also, why does anyone visit with a link including the "WebResource.axd?....." part in it?

Thanks,
Marina