Error , Never happen before

hi , i have few reports which encounter such things , have no idea what happen , 

Unexpected error: The specified key is not a valid size for this algorithm.
Parameter name: key Key length: 40
Details:
Microsoft.Mashup.Evaluator.Interface.ErrorException: The specified key is not a valid size for this algorithm.
Parameter name: key Key length: 40 ---> System.ArgumentException: The specified key is not a valid size for this algorithm.
Parameter name: key Key length: 40 ---> System.ArgumentException: The specified key is not a valid size for this algorithm.
Parameter name: key Key length: 40
at Microsoft.Mashup.EngineHost.Services.EncryptedPersistentCache.StreamEncryptor.EncryptPage(Byte[] input, Int32 count, Byte[] output)
at Microsoft.Mashup.EngineHost.Services.TransformingPagedStream.WritePage(Int32 page, Byte[] buffer, Int32 length)
at Microsoft.Mashup.EngineHost.Services.PagedStream.Flush()
at Microsoft.Mashup.EngineHost.Services.PagedStream.SeekToPage(Int32 page)
at Microsoft.Mashup.EngineHost.Services.PagedStream.Write(Byte[] array, Int32 offset, Int32 count)
at Microsoft.Mashup.EngineHost.Services.WriteBehindPersistentCache.AsyncCommitStorage(String key, TemporaryStorage storage)
at Microsoft.Mashup.EngineHost.Services.WriteBehindPersistentCache.AsyncCommitStorage(Object obj)
at Microsoft.Mashup.Evaluator.EvaluatorThreadPool.EvaluatorThread(Object state)
at Microsoft.Mashup.Evaluator.SafeThread2.<>c__DisplayClass9_0.<CreateAction>b__0(Object o)
--- End of inner exception stack trace ---
at Microsoft.Mashup.Evaluator.EvaluationHost.<>c__DisplayClass11_0.<TryReportException>b__1()
at Microsoft.Mashup.Common.SafeExceptions.IgnoreSafeExceptions(IEngineHost host, IHostTrace trace, Action action)
at Microsoft.Mashup.Evaluator.EvaluationHost.TryReportException(IHostTrace trace, IEngineHost engineHost, IMessageChannel channel, Exception exception)
at Microsoft.Mashup.Evaluator.EvaluationHost.TryHandleException(Exception exception)
at Microsoft.Mashup.Evaluator.SafeThread2.HandleException(Exception e)
at Microsoft.Mashup.Evaluator.SafeThread2.<>c__DisplayClass9_0.<CreateAction>b__0(Object o)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart(Object obj)
--- End of inner exception stack trace ---
at Microsoft.Mashup.Evaluator.EvaluationHost.OnException(IEngineHost engineHost, IMessageChannel channel, ExceptionMessage message)
at Microsoft.Mashup.Evaluator.MessageHandlers.TryDispatch(IMessageChannel channel, Message message)
at Microsoft.Mashup.Evaluator.MessageHandlers.Dispatch(IMessageChannel channel, Message message)
at Microsoft.Mashup.Evaluator.ChannelMessenger.ChannelMessageHandlers.TryDispatch(IMessageChannel channel, Message message)
at Microsoft.Mashup.Evaluator.MessageHandlers.Dispatch(IMessageChannel channel, Message message)
at Microsoft.Mashup.Evaluator.ChannelMessenger.OnMessageWithUnknownChannel(IMessageChannel baseChannel, MessageWithUnknownChannel messageWithUnknownChannel)
at Microsoft.Mashup.Evaluator.MessageHandlers.TryDispatch(IMessageChannel channel, Message message)
at Microsoft.Mashup.Evaluator.ChannelMessenger.ChannelMessageHandlers.TryDispatch(IMessageChannel channel, Message message)
at Microsoft.Mashup.Evaluator.MessageHandlers.Dispatch(IMessageChannel channel, Message message)
at Microsoft.Mashup.Evaluator.Interface.IMessageChannelExtensions.WaitFor[T](IMessageChannel channel)
at Microsoft.Mashup.Evaluator.RemotePreviewValueSource.PreviewValueSource.WaitFor(Func`1 condition, Boolean disposing)
at Microsoft.Mashup.Evaluator.RemotePreviewValueSource.PreviewValueSource.get_TableSource()
at Microsoft.Mashup.Evaluator.Interface.TracingPreviewValueSource.get_TableSource()
at Microsoft.Mashup.Host.Document.Analysis.PackageDocumentAnalysisInfo.PackagePartitionAnalysisInfo.SetPreviewValue(EvaluationResult2`1 result, Func`1 getStaleSince, Func`1 getSampled)