OAB Genration Failed

Hello, I have an Single multiple child domain controller scenario. Everything was working fine till day before, clients started reporting that they ain't recieving updated OAB in outlook. After some research in Exchange server OAB folder we found that folders OAB folder exists with its sub folders (a long GUID) but we were not able to see the same folders under IIS-->OAB virtual directory. Wwe have then manualy tried updating the OAB which gave an error below....                                                                                                                                                                                                                                                      " Generation of OAB "\Global_OAB" failed. Dn: CN=Global_OAB,CN=Offline Address Lists,CN=Address Lists Container,CN=Vedanta Exchange Org,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=vedantaresource,DC=local      
ObjectGuid: 932afc69-0712-4992-a9bd-57df6c0e02f5      
Stats: S:OAB='\Global_OAB';I64:Status=2147500037;Dt:StartTime=2015-04-01T06:15:23.8692958Z;Dt:EndTime=0001-01-01T00:00:00.0000000;S:DC=;I32:Total.Records=0;I32:Total.TempFiles=0;Ti:TimeWritingFiles=00:00:00;S:Org=vedantaresource.local;S:Wasted=False;I32:Total.RecordsAddedChurn=0;I32:Total.RecordsDeletedChurn=0;I32:Total.RecordsModifiedChurn=0;I32:MailboxDownload.FS.BytesRead=6211212;I32:MailboxDownload.FS.BytesWritten=6211212;Ti:MailboxDownload.FS.Reading.ElapsedTime=00:00:00.3238458;Ti:MailboxDownload.FS.Writing.ElapsedTime=00:00:00.1273409;Ti:PrepareFilesForOABGeneration.DownloadFilesFromMailbox.StoreRpcLatency=00:00:01.7970000;I32:PrepareFilesForOABGeneration.DownloadFilesFromMailbox.StoreRpcCount=1067;Ti:PrepareFilesForOABGeneration.DownloadFilesFromMailbox.CpuTime=00:00:03.1718750;Ti:PrepareFilesForOABGeneration.DownloadFilesFromMailbox.ElapsedTime=00:00:05.4275814;Ti:PrepareFilesForOABGeneration.CpuTime=00:00:03.1718750;Ti:PrepareFilesForOABGeneration.ElapsedTime=00:00:05.4277854;Ti:Total.CpuTime=00:00:03.1718750;Ti:Total.ElapsedTime=00:00:05.4278649;;      
S:Exp=System.UnauthorizedAccessException: Access to the path is denied.
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.File.InternalMove(String sourceFileName, String destFileName, Boolean checkHost)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGenerator.SafeFileReplace(String sourceFile, String destinationFile)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGenerator.DownloadFilesFromMailbox(MailboxSession mailboxSession)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGenerator.PrepareFilesForOABGeneration(AssistantTaskContext assistantTaskContext)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.<>c__DisplayClassc.<ProcessAssistantStep>b__8()
   at Microsoft.Exchange.Common.IL.ILUtil.DoTryFilterCatch(TryDelegate tryDelegate, FilterDelegate filterDelegate, CatchDelegate catchDelegate)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.ProcessAssistantStep(AssistantTaskContext assistantTaskContext) "                                                                                                                                                                                                                                                                                                                                                                          We do not understand why the problem has caused all of a sudden. Requesting help in this regards.                                                             

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Neilesh