soraphong,
Please add the following code into your web.config file then try again.
<httpHandlers>
<clear />
<add verb="*" path="*.ashx" type="System.Web.UI.SimpleHandlerFactory, System.Web, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
</httpHandlers>
Keep me posted
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