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The error occurs when the following plug-ins are disabled or not installed in Outlook:

  • Private Post Attachment Handler
  • Private Post v4

To resolve the issue:

  1. On Outlook 2007, go to Tools > Trust Center > Add-ins.
  2. Select Exchange Client Extensions from the Manage dropdown list, and then click Go.

    Exchange Client Extensions

  3. Make sure that the following options in the Add-In Manager window are enabled:
    • Private Post Attachment Handler
    • Private Post v4

    Add-In Manager window

  4. Click OK.
  5. Restart Outlook.