Unable to validate data.

[D Fields]

We get this when certain images are uploaded.....any ideas?........

Message:
Unable to validate data.

Stack Trace:
at System.Web.Configuration.MachineKey.GetDecodedData(Byte[] buf, Byte[]
modifier, Int32 start, Int32 length, Int32& dataLength) at
System.Web.UI.LosFormatter.Deserialize(String input)

Method Name:
GetDecodedData