KB5121003 Leaves ARM64 Users Without Teams and Outlook

KB5121003 reportedly breaks Microsoft Teams and New Outlook on some Windows 11 ARM64 PCs, with affected users saying both apps fail to launch after installing the August 2026 update.

The problem has been reported on devices including the Surface Laptop 7, Surface Pro 11, and HP OmniBook 3. Microsoft released KB5121003 as part of the August 2026 Patch Tuesday.

At the time of writing, Microsoft has not added the issue to the Windows 11 health dashboard.

Teams and New Outlook fail to launch

According to a Microsoft Answers report, the problem appears to mainly affect MSIX and AppX applications.

Users have specifically reported problems with Microsoft Teams and the New Outlook.

Traditional Win32 applications reportedly continue to work normally, suggesting that the issue may involve Windows’ modern app packaging infrastructure rather than ARM64 software in general.

System logs from affected PCs reportedly point to problems with AppModel Runtime package states. Users have also reported dependency failures and 0x80073CFC errors during token generation.

Core Windows packages such as MicrosoftWindows.Client.CBS may also play a role.

Uninstalling KB5121003 appears to fix the issue

So far, users say that removing KB5121003 restores Teams and New Outlook immediately.

For IT administrators managing ARM64 systems that rely on either application, pausing the update may therefore make sense until Microsoft investigates the problem or releases a fix.

Rolling back KB5121003 currently appears to be the most reliable reported workaround.

This is not the only problem linked to the update. KB5121003 is also reportedly causing crashes in ARC Raiders.

Via Neowin