Why do we get "An unexpected error occurred in Report Processing" in SSRS 2008?

We often see this kind of errors in SSRS log file. This seems like internal error happening inside SSRS engine. How can we debug this ? We are not getting any other relevant information when report fails.

library!ReportServer_0-1!2864!09/09/2016-00:28:15:: w WARN: Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: An unexpected error occurred in Report Processing. ---> System.Threading.ThreadAbortException: Thread was being aborted. at Microsoft.Win32.Win32Native.SetFilePointerWin32(SafeFileHandle handle, Int32 lo, Int32* hi, Int32 origin) at Microsoft.Win32.Win32Native.SetFilePointer(SafeFileHandle handle, Int64 offset, SeekOrigin origin, Int32& hr) at System.IO.FileStream.SeekCore(Int64 offset, SeekOrigin origin) at System.IO.FileStream.Seek(Int64 offset, SeekOrigin origin) at Microsoft.ReportingServices.Library.PartitionFileStream.Seek(Int64 offset, SeekOrigin origin) at Microsoft.ReportingServices.Library.MemoryUntilThresholdStream.Seek(Int64 offset, SeekOrigin origin) at Microsoft.ReportingServices.Library.RSStream.Seek(Int64 offset, SeekOrigin origin) at System.IO.BufferedStream.Seek(Int64 offset, SeekOrigin origin) at Microsoft.ReportingServices.Rendering.RPLProcessing.RPLReader.ReadElementProps(Int64 startOffset, RPLContext context, Byte& elementType) at Microsoft.ReportingServices.Rendering.ExcelRenderer.Layout.LayoutEngine.RenderNewItem(IRowItemStruct item, Int32 top, Int32 topRow, IExcelGenerator excel, String pageContentKey, Dictionary2 sharedBorderCache, Dictionary2 sharedImageCache, Boolean& autosizableGrow, Boolean& autosizableShrink) at Microsoft.ReportingServices.Rendering.ExcelRenderer.Layout.LayoutEngine.RenderPageToExcel(IExcelGenerator excel, String key, Dictionary2 sharedBorderCache, Dictionary2 sharedImageCache) at Microsoft.ReportingServices.Rendering.ExcelRenderer.MainEngine.RenderRPLPage(RPLReport report, Boolean headerInBody, Boolean suppressOutlines) at Microsoft.ReportingServices.Rendering.ExcelRenderer.ExcelRenderer.Render(Report report, NameValueCollection reportServerParameters, NameValueCollection deviceInfo, NameValueCollection clientCapabilities, Hashtable& renderProperties, CreateAndRegisterStream createAndRegisterStream) at Microsoft.ReportingServices.ReportProcessing.Execution.RenderReport.InvokeRenderer(IRenderingExtension renderer, Report report, NameValueCollection reportServerParameters, NameValueCollection deviceInfo, NameValueCollection clientCapabilities, Hashtable& renderProperties, CreateAndRegisterStream createAndRegisterStream) at Microsoft.ReportingServices.ReportProcessing.Execution.RenderReport.Execute(IRenderingExtension newRenderer) --- End of inner exception stack trace --- at Microsoft.ReportingServices.ReportProcessing.Execution.RenderReport.Execute(IRenderingExtension newRenderer) at Microsoft.ReportingServices.ReportProcessing.ReportProcessing.RenderReport(DateTime executionTimeStamp, ProcessingContext pc, RenderingContext rc, IChunkFactory yukonCompiledDefinition) at Microsoft.ReportingServices.Library.RenderLive.CallProcessingAndRendering(ProcessingContext pc, RenderingContext rc, OnDemandProcessingResult& result) at Microsoft.ReportingService webserver!ReportServer_0-1!2864!09/09/2016-00:28:16:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.UnhandledHttpApplicationException: , Microsoft.ReportingServices.Diagnostics.Utilities.UnhandledHttpApplicationException: The report server encountered an unhandled exception in HttpApplication. ---> System.Web.HttpException: Request timed out.
--- End of inner exception stack trace ---;

--------- UPDATE ----------

I see new error in log as

library!WindowsService_0!e88!09/09/2016-17:09:45:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerStorageException: , An error occurred within the report server database. This may be due to a connection failure, timeout or low disk condition within the database.; dbcleanup!WindowsService_0!e88!09/09/2016-17:09:45:: e ERROR: Error in CleanOrphanedSnapshots: Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerStorageException: An error occurred within the report server database. This may be due to a connection failure, timeout or low disk condition within the database. ---> System.Data.SqlClient.SqlException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. library!WindowsService_0!e88!09/09/2016-17:09:45:: i INFO: Cleaned 0 batch records, 0 policies, 0 sessions, 0 cache entries, 0 snapshots, 0 chunks, 4 running jobs, 0 persisted streams, 0 segments, 0 segment mappings, 0 edit sessions.

--------- UPDATE ----------

After updating all the timeout to 12hr and Put DO NOT TIMEOUT setting on SSRS site setting, we see diff error now.

So far we have checked execution log and it shows no error. Below error is found inside ssrs log file. Server has 32gb memory and we run on 64bit OS. This report sometimes is generated fine and its 200MB in size.

httpruntime!ReportServer_0-2!58dc!09/12/2016-23:26:45:: e ERROR: Failed in BaseWorkerRequest::SendHttpResponse(bool), exception=System.Runtime.InteropServices.COMException (0x800703E3): The I/O operation has been aborted because of either a thread exit or an application request. (Exception from HRESULT: 0x800703E3) at Microsoft.ReportingServices.HostingInterfaces.IRsHttpPipeline.SendResponse(Void* response, Boolean finalWrite, Boolean closeConn) at ReportingServicesHttpRuntime.BaseWorkerRequest.SendHttpResponse(Boolean finalFlush) library!ReportServer_0-2!58dc!09/12/2016-23:26:45:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerHttpRuntimeInternalException: RsWorkerRequest::FlushResponse., Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerHttpRuntimeInternalException: An internal or system error occurred in the HTTP Runtime object for application domain ReportServer_REPCENTER_0-2-131182052998828671. ---> System.Runtime.InteropServices.COMException (0x800703E3): The I/O operation has been aborted because of either a thread exit or an application request. (Exception from HRESULT: 0x800703E3) at ReportingServicesHttpRuntime.BaseWorkerRequest.SendHttpResponse(Boolean finalFlush) at ReportingServicesHttpRuntime.RsWorkerRequest.FlushResponse(Boolean finalFlush) --- End of inner exception stack trace ---;

Finally we have answer to this. Its the recyletime element inside "rsreportserver.config" file. By default this element value is set to 720 min (12 hr). This means from the time you start the SSRS service, it will recycle every 12 hr. Our service was started during mid night release this means it kept recycling everyday at noon. We have since update this value to 24 hr.

    <RecycleTime>1440</RecycleTime>
    <MaxAppDomainUnloadTime>60</MaxAppDomainUnloadTime>