Outlook Add-in becomes Inactive randomly

Question

Hi
We have created a VSTO Add-in for Outlook.  The add-in is intended as a plugin so that the other third parties can write their own "providers" that our addin can utilize.  As an example the add-in shows a grid of Items.  Anybody
can write a "provider" and display their own items in our grid.
To do this our add-in in made up of a three projects. core , common and library. The Library contains the public interfaces.
At run time the add-in uses MEF to load any provider assemblies.
This has been working quite successfully at numerous client sites with many users for some time.  However last month a client reported that when they start Outlook the Add-in was going inactive.  No attempts to re-active it would work it always
went back to inactive.
We turned on VSTO_SUPRESSDISPLAYALERTS and found that the issue seemed to be because the assemblies were not being loaded when the Add-in initialized.
Checking the assemblies that were being dynamically loaded we found that two of them were unsigned.  We subsequently signed the assemblies, re-applied and everything worked.
So far so good.  We patted ourselves on the back and made the assumption that some MS security Patch had been applied that stopped unsigned assemblies from being loaded at run time.
We could not however replicate the issue in house , not could we find the patch that might have caused the issue.
The problem however continues....
Our installations team took a draconian approach to this issue and started installing the signed assemblies on all our customer sites regardless of whether or not they had the original issue.
For most clients everything was fine.  The add-in worked with or without the signed assemblies.  However at one client site on a number of users pc's the add-in was still inactive.  removing the signed assemblies and putting back the unsigned
assemblies resolved the issue.  To say this is weird is an understatement. The complete opposite behavior.
Now this week they have reported another user has suddenly found the add-in is inactive and using neither the signed nor unsigned assemblies works.
We are at a complete loss what to look for next.
To summarize.
Our add-in dynamically loads assemblies at run time using MEF
It was working fine at all sites until recently.
Add-in randomly becomes inactive and cannot be re-activated either manually or through the registry.
Signing the Assemblies works at most sites
At one site unsigned assemblies work and signed assemblies do not
At the same site one user reports neither set of assemblies will activate.
We have run Caspol and cannot see anything amiss
We have used Caspol to apply Full Trust - no affect
We can not replicate in house
Things we are trying
Machine comparisons between working and non working pc's
Removal of all other add-ins
  If anyone has any other suggestions what could be causing such behavior it would be much appreciated.
Nick

Answer 1

Hello Nick,
I do not believe the issue depends on the signed/unsigned assemblies. Check out the Trust center settings to make sure that Macro security settings are not applied to add-ins. Also pay special attention to the Trusted Publishers list.
Anyway, Microsoft Office applications can disable add-ins that behave unexpectedly. If an application does not load your add-in, the application might have hard disabled or soft disabled your add-in.
Hard disabling can occur when an add-in causes the application to close unexpectedly. It might also occur on your development computer if you stop the debugger while the Startup event
handler in your add-in is executing.
Soft disabling can occur when an add-in produces an error that does not cause the application to unexpectedly close. For example, an application might soft disable an add-in if it throws an unhandled exception while the Startup event
handler is executing.
When you re-enable a soft-disabled add-in, the application immediately attempts to load the add-in. If the problem that initially caused the application to soft disable the add-in has not been fixed, the application will soft disable the add-in
again.
You can read more about that in the
How to: Re-enable an Add-in That Has Been Disabled article in MSDN. Most probably the add-in fires an exception at runtime and Outlook disables the add-in silently.
Also pay special attention to the
Performance criteria for keeping add-ins enabled . Outlook 2013 monitors add-in performance metrics such as add-in startup, shutdown, folder switch, item open, and invoke frequency. Outlook records the elapsed time in milliseconds for each performance monitoring
metric.
For example, the startup metric measures the time required by each connected add-in during Outlook startup. Outlook then computes the median startup time over 5 successive iterations. If the median startup time exceeds 1000 milliseconds (1 second),
then Outlook disables the add-in and displays a notification to the user that an add-in has been disabled. The user has the option of always enabling the add-in, in which case Outlook will not disable the add-in even if the add-in exceeds the 1000 millisecond
performance threshold