"Operation failed" error when accepting or proposing new meeting times in Outlook 2010

In Microsoft Outlook 2010, you may receive the following error when attempting to accept, or propose a new time for, a meeting request.

The operation failed. The messaging interfaces have returned an unknown error. If the problem persists, restart Outlook.

This scenario can occur if the following registry value is set:

Key: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\PST
DWORD: UseLegacyCacheSize
Value data: 1

This registry value may cause the issue when Outlook is running using a Cached Exchange Mode profile.

To resolve this issue, remove the UseLegacyCacheSize registry value from the Windows registry, using the steps below.

Important Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.

  1. Exit Outlook.
  2. Start Registry Editor. To do this, use one of the following methods, as appropriate for your version of Windows:
    • Windows 8: Press the Windows Key + R to open a Run dialog box. Type regedit.exe, and then press OK.
    • Windows 7: Click Start, type regedit.exe in the search box, and then press Enter.
  3. In the Registry Editor, locate the following subkey in the registry:


  4. Locate and right-click the following value:


  5. Click Delete, and then click Yes to confirm the deletion.
  6. Exit Registry Editor.

More information
If you review the Microsoft Office Alerts log, you will see an event similar to the following example:

Microsoft Outlook

The operation failed.  The messaging interfaces have returned an unknown error. If the problem persists, restart Outlook.

P1: 301670
P2: 14.0.4763.1000
P3: 6xk3
P4: 0x80040811

To review the Microsoft Office Alerts log, follow these steps:
  1. In Control Panel, open Administrative Tools.
  2. Double-click Event Viewer.
  3. In the navigation pane, select Microsoft Office Alerts under Applications and Services Logs.

Note This is a "FAST PUBLISH" article created directly from within the Microsoft support organization. The information contained herein is provided as-is in response to emerging issues. As a result of the speed in making it available, the materials may include typographical errors and may be revised at any time without notice. See Terms of Use for other considerations.

Article ID: 3096065 - Last Review: 09/11/2015 22:10:00 - Revision: 2.0

Microsoft Outlook 2010

  • KB3096065