Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 504

I get the following message soon after 120 seconds. Report query returns the full data set in SSMS withing 25 seconds.

This is a fairly a large report. I get this same error messages many of my large data reports.

Report server version 11.0.5058.0 on a VMware 2 GHz 4 core, 64GB RAM and Windows 2012 R2 (64 bit)

DB Server version Standard Edition (64-bit) 11.0.5058.0 on a VMware 2 GHz 8 core, 96GB RAM and Windows 2012 R2 (64 bit)

library!ReportServer_0-2!15ec!02/24/2015-14:29:07:: i INFO: Using folder C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\RSTempFiles for temporary files.
webserver!ReportServer_0-2!15ec!02/24/2015-14:29:17:: i INFO: Processed report. Report='/Head Office/UAE/RSH/Free Stock Report - Item wise', Stream=''
rshost!rshost!1404!02/24/2015-14:29:46:: e ERROR: WriteCallback(): failed to write in write callback.
rshost!rshost!1404!02/24/2015-14:29:46:: e ERROR: Failed with win32 error 0x03E3, pipeline=0x00000006101063B0.
rshost!rshost!1684!02/24/2015-14:29:46:: e ERROR: HttpPipelineCallback::SendResponse(): failed async writing response.
rshost!rshost!1684!02/24/2015-14:29:46:: e ERROR: Failed with win32 error 0x03E3, pipeline=0x00000006101063B0.
httpruntime!ReportManager_0-1!1684!02/24/2015-14:29:46:: 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!ReportManager_0-1!1684!02/24/2015-14:29:46:: 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 ReportManager_MSSQLSERVER_0-1-130692470239287110.  ---> 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 ---;

I get the following error in visual studio