When there is a Microsoft Exchange account in the Outlook, and the PC doesn't have any available network connections (i.e. completely off-line, rather than Outlook working in off-line mode), COMException will raise when checking for the folder information.

To be more exact, COMException (0x8834010F) will raise in ThisAddIn.RefreshFolderMonitors.VisitFolder when accessing the property of 1 Folder of the Folders collection of the Microsoft Exchange root folder. And COMException (0x0xA304010F) will raise in MonitoredFoldersDialog.ShowDialog.PopulateFolders for the similar operation.

For now, I simply suppressed the exception, but it is still open to further investigations.