For some weeks now we have been fighting with an issue where at a small number of customers our Outlook addin gets unloaded and disabled for yet undetermined reasons. By "disabled" I mean that Outlook changes the following registry value from 3 to 2 which in effect means that the addin will not be loaded on next startup:

HKEY_LOCAL_MACHINE\Software\Microsoft\Office\Outlook\Addins\[OurAddin.sProgID]\LoadBehavior

There is no error message and neither do any exceptions show up in the log files that our addin produces itself.

I have already found the following page which specifically deals with the LoadBehavior change issue: http://blogs.msdn.com/vsod/archive/2008/04/22/Troubleshooting-com-add-in-load-failures.aspx

However, none of the possible reasons proposed there appear to be applicable:

  • The addin is not merely listed in the Disabled Items list.
  • There are no unhandled exceptions neither in the IDTExtensibility2 methods nor anywhere else in the code. All code is wrapped in try/catch equivalents and all exception output is emitted only via OutputDebugString or into a log file.
  • The error appears to be independent of anti-virus software, i.e. it also occurs with it disabled.
  • Disabling all other addins also has no effect on the error.

So, what else can cause Outlook to disable an addin?

Some more details / observations:

  • We haven't been able to reproduce the issue in our test environments so far so we haven't yet been able to attach a debugger while the issue occurs.
  • The issue never occurs while we try to watch what happens via remote support (TeamViewer). I suspect this is because TeamViewer uses a hook DLL that injects itself into all running processes (including Outlook) a
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