Add-ins stop working in Outlook on the web for Exchange Server 2016 RTM

Starting on May 3, 2021, you can't start add-ins in an instance of Outlook on the web that's running on Microsoft Exchange Server 2016 RTM.

This change occurs because the Outlook add-ins use the Office JavaScript APIs that are accessed through the Office JS (Office.js) Content Delivery Network (CDN) to interact with content in Outlook on the web. These JavaScript APIs are deprecated for Exchange Server 2016 RTM.

After this change is made, you receive the following error message when you try to open an add-in in Outlook on the web:

This add-in has been disabled to help keep you safe.

Screenshot of an error message - this add-in has been disabled to help you keep safe.

To resolve this issue, upgrade the Exchange-based server to the latest cumulative update.