you should be doing this in code:

If DotNetNuke.Framework.AJAX.IsInstalled Then
    DotNetNuke.Framework.AJAX.RegisterScriptManager()
End If

see here for more info: http://adefwebserver.com/DotNetNukeHELP/Misc/Dotnetnuke_AJAX_modulei.htm

You cannot register it with the ScriptManager control, because only one scriptmanager is allowed in the page. If you put two modules in the page that contain scriptmanagers, that will cause an error

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i am having the same issue.  this is my code in page_load:

  If DotNetNuke.Framework.AJAX.IsInstalled Then
                    DotNetNuke.Framework.AJAX.RegisterScriptManager()
                    Dim objScriptManager As ScriptManager = ScriptManager.GetCurrent(Me.Page)
                    Dim objServiceReference As ServiceReference = New ServiceReference
                    objServiceReference.Path = "~\DesktopModules\DealerInventoryDetailPhoto\MRVASlideshow.asmx"
                    objScriptManager.Services.Add(objServiceReference)
                End If

it is a ms ajax slideshowextender.  my module passed testing and was installed on a live site with several hundred portals.  it worked fine all day and then in the afternoon it started blowing up ( The control with ID 'slideshowextend1' requires a ScriptManager on the page. The ScriptManager must appear before any controls that need it.) and blew on all portals until the app pool was reset.   it worked fine for several hours and then it happened again.  it worked through the night and then this morning it started erroring about 20 minutes after our daily app restart.  it seems very indiscriminate but once it starts blowing it will continue to do so until we do a restart.

it there anyway that the DotNetNuke.Framework.AJAX could fail to install?

thanks.