alternate access mapping in sharepoint 2010‎+Forefront TMG 2010 have an error via internal URL

Hi guys

I have finished alternate access mapping configuration in sharepoint 2010‎‎ + Forefront TMG 2010. But if I access my SharePoint website via internal URL, everything is working fine. But if use external via TMG server access to my website. Got a JavaScript error and all of ajax post back can not work. 

I have try to create a simple asp.net ajax website, Use the same URL and port number. I can access and all the ajax action is working fine. So i think the problem should be in SharePoint, Maybe I missed some configuration. Following is my error message.

Uncaught Sys.ScriptLoadFailedException: Sys.ScriptLoadFailedException: The script 'https://MyExternalSite/ScriptResource.axd?d=NE3UDhnLSd6GpGqvQ9UCl_VOolCiNPjfrcsX2qm--dIu9tVpW1aaWGIImzfG0hDh3db7SlPYtvUJNy7jBVx-tU3ZTDWaE-L4ZIRW-jcztHYKw9Ib0tAzmb7sUn0d2T4l-AUVf4kMZVK0cuXOc2ppNktOBF01&t=ffffffffb868b5f4' could not be loaded. ScriptResource.axd:5
Error.create ScriptResource.axd:5
Sys._ScriptLoader._errorScriptLoadFailed ScriptResource.axd:5
Sys.WebForms.PageRequestManager._scriptIncludesLoadComplete ScriptResource.axd:5
Sys.WebForms.PageRequestManager._scriptIncludesLoadFailed ScriptResource.axd:5
(anonymous function) ScriptResource.axd:5
(anonymous function) ScriptResource.axd:5
Sys._ScriptLoader._raiseError ScriptResource.axd:5
Sys._ScriptLoader._scriptLoadedHandler ScriptResource.axd:5
(anonymous function) ScriptResource.axd:5
(anonymous function) ScriptResource.axd:5

Thanks

Dayan