The solution depends on a combination of the version of your Windows OS and your Outlook which determines the browser used to render all of your add-ins (this is entirely different to your default browser and will either be Internet Explorer or Edge).
Refer to this Microsoft article Browsers used by Office Add-ins to determine if your browser is IE or Edge.
If your browser is determined as Edge
Users on updated versions of Windows 10 and Microsoft 365 typically have Edge used to render their Add-ins and it is possible that the Desktop App Web Viewer is corrupted. Take the steps below to reinstall it.
- Close all Office modules including Outlook.
- Run the following command on your machine with elevated privileges.
powershell Add-AppxPackage -register -disable c:\windows\SystemApps\Microsoft.Win32WebViewHost_cw5n1h2txyewy\AppxManifest.xml
- Restart any Office module and load the FYI add-in to check the issue is resolved
If your browser is Internet Explorer
Microsoft renders Add-ins using Internet Explorer for users on older versions of Windows and Outlook.
As per the Microsoft article referenced above, users should upgrade to Windows 10 version 1903 or higher and Microsoft 365 version 16.011629 and higher.
Enable Protected Mode in the Internet Properties
If upgrading is not an option, then enable protected mode in Internet Explorer. Even though your default browser is not Internet Explorer note that Microsoft leverages Internet Explorer for users on specific combinations of Windows and Office versions.
- Close Outlook and all your browsers.
- Open Internet Explorer.
- Click Tools.
- Click Internet Options.
- Click Security tab.
- Select Internet Sites zone.
- Select Enable Protected Mode (make sure box is checked).
- Click OK.
- Restart Internet Explorer and Outlook.
- Try to reproduce the issue.
- If the issue persists reboot your computer and try again.