To preserve the content-type when the SharePoint server reports HTTP errors to Forefront Unified Access Gateway (UAG), you must configure the IIS server on Forefront UAG to return detailed errors, as described in the following procedure.
To configure error reporting
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On the Forefront UAG server, click Start, and then in the Start Search box, type inetmgr and press ENTER.
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In the Internet Information Services (IIS) Manager, in the navigation tree, under Sites, click the site name corresponding to your portal.
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In the center pane, in the IIS section, double-click Error Pages.
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In the Actions pane, click Edit Feature Settings.
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On the Edit Error Pages Settings dialog box, in the Error Responses area, click Detailed errors, and then click OK.