You may receive intermittent HTTP 500 errors when you try to access web applications through Microsoft Forefront Unified Access Gateway 2010 if you are not authorized to access the application. For example, this issue may occur when you are not a member of an authorized group or when you do not meet the Unified Access Gateway endpoint access policy for the application.
If you update the webpage, the HTTP 500 error may be replaced with the correct "You are not authorized" error page. This issue is typically seen on requests to a published Outlook Web App (OWA) application. But this issue may also affect other web applications. This issue occurs only if you publish Outlook Anywhere or Microsoft Exchange ActiveSync on the same trunk.
This issue occurs because Unified Access Gateway maintains a pool of objects that are used to store information about incoming requests. When a new request arrives at the Unified Access Gateway server, Unified Access Gateway takes an object from the pool and may read property values from the object. One of the properties defines whether the client application for the request can support HTML-rendered error pages such as for all web browsers, or the client application can support only HTTP error code values such as for Microsoft Outlook or Exchange ActiveSync. In some circumstances, the property is not reset correctly when an object is returned to the pool. If a new OWA request uses an object that was previously used to handle an Outlook Anywhere or Exchange ActiveSync request, Unified Access Gateway may incorrectly return an HTTP Status code to the browser because the property value is incorrect.
To resolve this problem, install the service pack that is described in the following Microsoft Knowledge Base article:
2710791 Description of Service Pack 2 for Forefront Unified Access Gateway 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates