Below is the log when it fails. Have tried 9.02 and 9.03 client without change. This happens when the user reboots and returns the following morning. The add-in must be repaired or re-installed to restore the tab in Outlook.
23/02/2012 12:30:36.424[3288][H]: Log file created (level 3).
23/02/2012 12:30:36.424[3288][L]: FUIHelper::FUIHelper: 0x0
23/02/2012 12:30:36.426[3288][L]: ~FUIHelper::FUIHelper: 0x0
23/02/2012 12:30:36.426[3288][L]: CDesktopUI::CDesktopUI
23/02/2012 12:30:36.427[3288][L]: ~CDesktopUI::CDesktopUI
23/02/2012 12:30:36.428[3288][L]: CDesktopUI::OnConnection: 0x0
23/02/2012 12:30:36.431[3288][L]: CDesktop::FinalConstruct: 0x0
23/02/2012 12:30:36.431[3288][L]: DesktopCommonConfig::LogonToMAPI: 0x0
23/02/2012 12:30:36.441[3288][L]: ~DesktopCommonConfig::LogonToMAPI: 0x80040111
23/02/2012 12:30:36.442[3288][M]: Connections now on Desktop object: 1
23/02/2012 12:30:36.443[3288][L]: ~CDesktop::FinalConstruct: 0x80040111
23/02/2012 12:30:36.443[3288][L]: CDesktop::FinalRelease
23/02/2012 12:30:36.443[3288][M]: Connections left on Desktop object: 0
23/02/2012 12:30:36.444[3288][L]: DesktopCommonConfig::ReleaseStatics: 0x0
23/02/2012 12:30:36.445[3288][L]: ~DesktopCommonConfig::ReleaseStatics: 0x0
23/02/2012 12:30:36.445[3288][M]: Disconnecting...Clearing MAPI
23/02/2012 12:30:36.446[3288][L]: DesktopCommonConfig::LogOffMAPI: 0x0
23/02/2012 12:30:36.446[3288][L]: ~DesktopCommonConfig::LogOffMAPI: 0x0
23/02/2012 12:30:36.447[3288][M]: Released Session
23/02/2012 12:30:36.447[3288][M]: Closing event handles
23/02/2012 12:30:36.447[3288][L]: DesktopCommonConfig::CloseDown: 0x0
23/02/2012 12:30:36.448[3288][L]: ~DesktopCommonConfig::CloseDown: 0x0
23/02/2012 12:30:36.448[3288][M]: ...Disconnected
23/02/2012 12:30:36.449[3288][L]: ~CDesktop::FinalRelease
23/02/2012 12:30:36.449[3288][H]: A COM error exception has been caught: 80040111
23/02/2012 12:30:36.450[3288][L]: ~CDesktopUI::OnConnection: 0x80040111
