Consider the following scenario:
You configure the proxy server connection settings for Windows Internet Explorer.
You add the Client Access Server (CAS) servers to the proxy exceptions list in Windows Internet Explorer on a client computer.
You open a mailbox profile in Microsoft Outlook 2010 by using credentials that are different to the credentials that you used to log on to Windows.
You try to open the Out of Office Assistant in Outlook 2010.
In this situation, you receive the following error message:
Your Out of Office settings cannot be displayed, because the server is currently unavailable. Try again later.
Note You do not receive this error message in either of the following scenarios:
You log on to Windows by using the local user credentials.
You log on to Windows as a domain user, but you do not add the CAS servers to the proxy exceptions list in Windows Internet Explorer.
This problem occurs because the CAS server uses the credentials that you used to log on to Windows instead of the credentials that you used to log on to the mailbox in Outlook 2010. Because the Windows credentials do not match the mailbox credentials, the CAS server returns a "500" error code. However, the "500" error code is not an authentication error code. Therefore, no other authentication methods are tried.
To resolve this issue, apply the following update:
2584053 Description of the Outlook 2010 hotfix package (x86 Outlook-x-none.msp; x64 Outlook-x-none.msp): August 30, 2011
Registry key information
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in WindowsAfter you apply the hotfix package, follow these steps to enable the hotfix:
Click Start, click Run, type regedit in the Open box, and then click OK.
Locate and then click the following registry subkey:
On the Edit menu, point to New, and then click DWORD (32-bit) Value.
Type WinhttpAuth, and then press ENTER.
In the Details pane, right-click WinhttpAuth, and then click Modify.
In the Value data box, type 1, and then click OK.
Exit Registry Editor.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.