ADD-IN ERROR - This add-in cloud not be started. - CRITICAL

Question

I have notice this error happens for ALL modern Outlook Office Add-ins. Searching the net I can also see that many other Modern Outlook Add-ins are having the same issue. We know that this is an Outlook Client issue. You never get the error on OWA.

I have also seen that Microsoft are suggesting

1. Open Internet Explorer
2. Click Tools
3. Click Internet Options
4. Click Security tab
5. Select Internet Sites zone
6. Select Enable Protected Mode (make sure box is un checked)
7. Click OK
8. Restart Internet Explorer and Outlook

However, we cannot ask customer to do this. Many are not even using IE. And it doesn't work most of the times!

In our case the Add-in service is running on Azure. We have enabled application insight and are monitoring failures on a daily basis. Our service is running on 20% of the avilable capacity. I feel that we have done all that we could. We are stock! and very frustrated.

Logically, when the Add-in works with no error in OWA, and only fails in the Outlook client this can only mean the error is in Outlook or windows or.... meaning it is a Microsoft error.

Answer 1

This error is by-design for add-ins in Outlook clients that are rendered using IE and do not have Protected Mode enabled. Protected Mode is a security feature when using IE, and it is strongly recommend that it be enabled while using add-ins.

If you do not or cannot enable Protected Mode, a possible workaround is to upgrade to a newer Windows and Office, so add-ins are rendered using Edge Webview instead of IE. Details about what versions that Webview are supported can be found here.