Outlook 2016 Add-In doesn't work for some users - outlook

Question

We've created an Outlook add-in which integrates our custom CRM with Outlook. A command button is added to messages in read and compose mode. A function file is called on the button click.
The add-in manifest passes validation and installs properly on all machines. However on Outlook 2016 desktop (only) we are seeing an error on some machines. The exact error message says: “We’re sorry, we couldn’t access [NCS Outlook Add-in]. Make sure you have a network connection. If the problem continues, please try again later.” ([NCS Outlook Add-In] is the name of our add-in.)
Note that this message appears IMMEDIATELY after clicking the add-in command button. It does not first say “[NCS Outlook Add-in] is working on your request” like it add-ins do when an error has occurred.
We have tried Outlook logging and haven't found anything useful in the logs. We've checked settings and disabled other add-ins. We have also tested across machines with user accounts. The same user will have the error on one machine and not the other. In short, it seems that the error is machine specific and not profile related.
Version of Office 365 installed: 1705 (Build 8201.2209).

Answer 1

We experienced this exact behavior in Outlook 2016, the solution was to enable protected mode for the restricted and internet zones.
The easiest way to change this is in IE.
In Internet Explorer, click the Tools button, and then click Internet Options.
Click the Security tab, and then select the Restricted Sites Zone.
Select the Enable Protected Mode check box, and then click OK.
Restart Internet Explorer.
Here is a little more information:
https://support.microsoft.com/en-us/help/2761180/apps-for-office-don-t-start-if-you-disable-protected-mode-for-the-rest

Answer 2

Try this:
1) Close Outlook
2) Rename the folder C:\Users\%USERNAME%\AppData\Local\Microsoft\Outlook
to C:\Users\%USERNAME%\AppData\Local\Microsoft\Outlook.OLD
(Where %USERNAME% is current User Name.)
3) Start Outlook, Setup User Profile
4) Add/Test Add-In.
If that works, then something broken on old Outlook profile. But if same problem: Exit Outlook, delete the Outlook folder and rename Outlook.OLD back to Outlook.

Answer 3

I ran into this exact same problem after uninstalling Visual Studio 2015 on my development machine. (I had been using Visual Studio 2017 for Outlook add-in development.)
The problem went away as soon as I reinstalled Visual Studio 2017. The Outlook 2016 installation had nothing to do with the problem, I never changed Outlook or its profile at all.
This would indicate that the problem is somehow related to the SDK tooling on the machine...perhaps something to do with the .Net framework?
If I ran into this problem on a customer machine I would try reinstalling or repairing the .Net framework, or perhaps reinstalling Outlook (assuming it contains some add-in tooling that is broken.)
Just passing this on in case it helps anyone else.