Problem with running SSRS Reports thorugh ISA2006

I have been struggling with an issue for a few days at present.
Our scenario
We previously had a working ISA and sharepoint/SSRS combination that all worked fine for reports.  We are currently migrating the SSRS server to a new platform.

We have created a new SSRS platform and updated the ISA links as appropriate.
When we run the reports though ISA, the reports all fail after the first post back with;
" 
  Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed.  Common
  causes for this error are when the response is modified by calls to Reponse.Write(), response filters, HttpModules, or server trace is
  enabled.
  Details: Error parsing near '...some random HyperText that changes per report'
"
Internally however the reports work fine.  The reports previously worked on old SSRS platform going through ISA.

From what I gather, this is likely an Ajax postback related issue.  I can not seem to find a resolution.
Most references about this error are to do with ssrs directly and not anything to do with when using ISA.  Our problem only appears when using ISA.  I read about MaxHttpCollectionKeys being a possible cause so I tweaked that but it did nothing.
I have tried changing Report Paramaters to Never Refresh, but makes no difference.

A lot of talk is about changing AsyncRender=false.  This does not appear to make any difference in this scenario.
Several other things tried such a
validateRequest="false" enableEventValidation="false" enableViewStateMax="false" enablePartialRendering="false"
 in the web.config on the ssrs, but nothing appears to make any difference.  

Our setup is;
ISA2006 TMG
   - Link translation > Configure > Replace /ReportServer with https://ourexternalurl.com/ourNEWssrsservername/ReportServer
SSRS Server - Windows Server 2012 R2 with MS SQL Server2014 SSRS
Reports have been updated /deployed with BIDS Visual Studio 2008
 
Previous setup was;
ISA2006 TMG
  - Link translation > Configure > Replace /ReportServer with https://ourexternalurl.com/ourOLDssrsservername/ReportServer
SSRS Server - Windows Server 2003 SP2 with MS SQL Server 2005 SSRS
Previous deployment/build of reports published using BIDS Visual Studio 2005

I have re-published on VS2005 build of reports to new server but they fail in same way so have confidence its not to do with the bids VS2008 publishing environment.

Looking for any ideas to try.

Regards
Jason