Outlook for Windows bug in version 1.2026.1020.100

Microsoft deployed a WebView2 kill-switch for this attachment-opening regression, so fully terminate and relaunch new Outlook, allow up to 6–7 hours for the rollout to apply, and use classic Outlook or Save/Download if the failure persists.

Verify and apply the deployed fix

1. In new Outlook → Settings → General → About Outlook, record:
   • Microsoft Outlook version
   • Client version
   • WebView2 version
2. If WebView2 is 156.x, the symptoms match the reported regression: attachments fail to open in desktop applications, Outlook displays “Can’t open the file,” and the Open action may disappear. The issue was reproduced with WebView2 156.0.4314.42 and 156.0.4314.50; 155.0.4283.62 did not exhibit it in the reported A/B test.^1^
3. Completely close new Outlook:
   • Quit it from the system tray if present.
   • In Task Manager, end any remaining Outlook (new)/olk.exe processes.
   • Relaunch Outlook and retest opening an attachment in its default desktop app.

Microsoft reported deploying a kill-switch through the current stable configuration and said it should apply within 6–7 hours after relaunch. Initial affected users later confirmed recovery, but a newer report says some systems remained affected on WebView2 156.0.4314.58. Therefore, continued failure does not necessarily mean the rollout has simply not arrived; another runtime state, policy, attachment type, or unresolved trigger may be involved.^1^

There is no documented public rollback procedure specifically for new Outlook 1.2026.1020.100. Do not delete or rename WebView2 DLLs or manually replace the Evergreen runtime; those community modifications are unsupported and may be undone by servicing.

Supported workarounds

Until the affected systems recover:

• Use classic Outlook to open attachments. Microsoft supports running classic and new Outlook side by side.^2^
• Alternatively, use the attachment menu’s Save as/Download, then open the saved file from File Explorer.
• If available, use Open in browser/web app.

If the option to return to classic Outlook is hidden by policy, an Exchange administrator can expose it by setting:

Set-OwaMailboxPolicy -Identity OwaMailboxPolicy-Default `
  -HideClassicOutlookToggleOut $false

This changes the mailbox policy controlling whether the classic-Outlook toggle appears.^3^

If the problem remains

1. Check the Microsoft 365 admin center for a current Outlook advisory and Microsoft’s public new-Outlook known-issues page.^4^
2. Open a Microsoft 365 support request and include:
   • Outlook, client, and WebView2 versions
   • Windows and Office builds
   • Affected attachment types
   • Whether Save/Download, Outlook on the web, and classic Outlook work
   • Relevant Edge/WebView2, Intune, DLP, and endpoint-security policies
   • The fact that the issue followed the new-Outlook update onto replacement hardware
3. If WebView2 is not 156.x, check %userprofile%\AppData\Local\Microsoft\Olk. If the Attachments subfolder is absent, create it, relaunch Outlook, and retest. A matching new-Outlook attachment failure was resolved this way, but it is a fallback rather than the primary explanation for the reported cross-device regression.^5^

References

1. #5782 Problem/Bug: WebView2 Runtime 156.x prevents New Outlook attachments from opening, Fixed Runtime 155.0.4283.62 works
2. Deployment overview for the new Outlook for Windows
3. Policy for Admin-Controlled Migration to new Outlook for Windows
4. Fixes or workarounds for recent issues in new Outlook for Windows ...
5. How to fix error occurred while opening the file on outlook (new) - Microsoft Q&A