ADD-IN ERROR (due to corrupted Legacy Edge) – Hoxhunt

Sometimes the ADD-IN ERROR may persist, even after checking all settings according to this article.

This situation is caused by a corrupted Legacy Edge runtime.

IMPORTANT: MS Edge Chromium will be installed to practically all Win 10 versions (1803 or later) as part of a Security update in April 2021. Legacy Edge will be automatically uninstalled, however the EdgeHTML runtime should stay ”intact”.
Read more: Which Windows 10 versions will be impacted?

ADD-IN ERROR
This add-in could not be started. Close this dialog to ignore the problem or click "Restart" to try again.

Occurs in: Windows desktop Outlook client (Microsoft 365/MSO/Click-to-Run, Outlook 2016, 2019)

Doesn't occur: Web Outlook (OWA), any web browser

Reference: https://github.com/OfficeDev/office-js/issues/1620

Details: Win32WebViewHost.exe crashes and causes ADD-IN error to appear.

Steps that may lead to the error scenario:

Installing Chromium Edge (WebView2) over Legacy Edge corrupts Legacy Edge’s runtime engine (Edge HTML, WebView).
AND
User’s Office version/build is high enough to use Edge HTML (lower than 16.0.13530.20424) but not high enough to use Edge Chromium (16.0.13530.20424 or higher).

Recommended fix:

Close all Office applications (especially Outlook).
Rename this folder:
%userprofile%\AppData\Local\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\
Execute the following Powershell (with user's permissions):
Get-AppxPackage "Microsoft.Win32WebViewHost" | foreach {Add-AppPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}

If the fix worked, you can remove the renamed folder from Step 1.
If the fix didn't work, move to the alternative fix below.

Alternative fix:
(kudos to DB-ADM!)

The permissions for WebView's AppData folder might have gotten corrupted or overwritten. Here's how to tell if that's the case:

Navigate to %localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy
In that directory, grant "ALL APPLICATION PACKAGES" full control of that folder, subfolders, and files (If domain-joined, make sure the local machine is selected for the location when searching for the group).
Now try launching the add-in. If it works, great!

You'll want to apply the correct permissions now.

Remove the permissions that were given above.
Run the following from an elevated command prompt:

icacls "%localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\AC" /grant "*S-1-15-2-1310292540-1029022339-4008023048-2190398717-53961996-4257829345-603366646:(F)" /T
icacls "%localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\AC\#!123" /grant "*S-1-15-2-1310292540-1029022339-4008023048-2190398717-53961996-4257829345-603366646-3829197285-1050560373-949424154-522343454:(F)" /T
icacls "%localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\LocalCache" /grant "*S-1-15-2-1310292540-1029022339-4008023048-2190398717-53961996-4257829345-603366646:(F)" /T
icacls "%localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\LocalState" /grant "*S-1-15-2-1310292540-1029022339-4008023048-2190398717-53961996-4257829345-603366646:(F)" /T
icacls "%localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\RoamingState" /grant "*S-1-15-2-1310292540-1029022339-4008023048-2190398717-53961996-4257829345-603366646:(F)" /T
icacls "%localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\Settings" /grant "*S-1-15-2-1310292540-1029022339-4008023048-2190398717-53961996-4257829345-603366646:(F)" /T
icacls "%localappdata%\Packages\Microsoft.Win32WebViewHost_cw5n1h2txyewy\TempState" /grant "*S-1-15-2-1310292540-1029022339-4008023048-2190398717-53961996-4257829345-603366646:(F)" /T

After granting those permissions, add-in should now work.

Notes from AD-ABM:
The SID ended in 6646 is the Win32WebViewHost Application Container. I'm not certain what the longer version of this SID belongs to, but I found it present on my test workstation so I added it in as well. I had to go through and manually enumerate the permissions in these directories so I'm hoping I'm not missing anything.

Additional reports of this issue:

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