Teams and New Outlook Won't Open on Windows on Arm After KB5121003

Microsoft has scoped this narrowly. A wrong diagnosis here ends with you uninstalling a security update you never needed to remove.

Applies to: Windows 11 26H1, 25H2 and 24H2 on Arm64

1. Check the processor. Open Settings, then System, then About, and read Device specifications. Microsoft lists this issue for Arm-based devices only — a Snapdragon or other Arm chip. Intel and AMD machines are not on its affected list.
2. Check the build. Press Windows+R, type winver and press Enter. On Windows 11 24H2 or 25H2 the affected builds are 26100.9168 or 26200.9168, installed as KB5121003. On Windows 11 26H1 — which ships preinstalled on some new Snapdragon PCs and is never offered through Windows Update — the same issue arrives as KB5121000, OS Build 28000.2704. If you went looking for KB5121003 in your update history on a 26H1 machine and could not find it, that is why.

   winver

3. If winver already shows build 26100.9445, 26200.9445 or 28000.2954 or higher, the September update carrying the fix is installed. Teams still failing on those builds is a different problem — see "When something else is wrong" below.
4. Check which apps fail. Microsoft's issue covers Microsoft Teams and the new Outlook for Windows. If classic Outlook, Word and Excel start normally, that matches. If everything is failing, or the taskbar and Start search are broken too, you are looking at something wider than this known issue.

Confirmed by the vendor.

Source: Microsoft, Microsoft, Microsoft, Microsoft, Microsoft

This is Microsoft's actual fix, not a workaround. Both KB articles state that the update addresses Microsoft Teams and Microsoft Outlook unexpectedly closing on Arm64-based PCs, and Microsoft marked the known issue Resolved on the release-health pages for 24H2, 25H2 and 26H1 on the day they shipped.

Applies to: Windows 11 26H1, 25H2 and 24H2 on Arm64

1. Open Settings, then Windows Update, and click Check for updates.
2. Install the September 2026 security update. On Windows 11 24H2 and 25H2 it is KB5124008, taking the build to 26100.9445 or 26200.9445. On Windows 11 26H1 it is a different update, KB5124012, taking the build to 28000.2954. Windows offers the right one for your version — you do not need to choose.
3. Restart when prompted, then confirm the build with winver.

   winver

4. Start Microsoft Teams and the new Outlook. Microsoft states that once you are on an update released 8 September 2026 or later, no workaround is needed.
5. If updates are paused or deferred, clear that first in Settings > Windows Update, or the September update will not be offered.

Confirmed by the vendor.

Source: Microsoft, Microsoft, Microsoft, Microsoft

This was Microsoft's interim workaround, published on 1 September 2026, and Microsoft still documents it for anyone on an update released before 8 September. It is second choice now that a real fix exists, but it is useful on a managed or deferred machine that cannot take the September update today.

Applies to: Windows 11 26H1, 25H2 and 24H2 on Arm64

1. Open the Microsoft Store.
2. Select Downloads, then Check for updates. On a new or freshly reset PC this is often a long list; let it finish rather than stopping at the first few.
3. Make sure the Auto Super Resolution Package is among the apps that install, and that it reaches version 1.0.19.0 or later. Microsoft links to it in the Store as an app published by Microsoft.
4. Start Teams or the new Outlook again.
5. Come back to Windows Update when you can. The September update is the fix; this only works around the problem.

Confirmed by the vendor.

Source: Microsoft, Microsoft

Not a fix, a way to keep working while the September update installs. Microsoft states classic Outlook, Word and other applications are not known to be affected, so mail on the same machine still works.

Applies to: Windows 11 26H1, 25H2 and 24H2 on Arm64

1. Open Outlook from the Start menu and check whether it is the classic desktop app rather than the new Outlook — classic Outlook is a separate application and Microsoft does not list it as affected.
2. If your organisation has already removed classic Outlook, use Outlook on the web instead until the September update is on the machine.

Source: Microsoft

This was the community's answer through late August and it does work, but it is now the wrong move in nearly every case: the September update fixes the problem without giving up any security fixes. It is documented here only for a machine that can neither take the September update nor the Store workaround.

Applies to: Windows 11 26H1, 25H2 and 24H2 on Arm64

1. Try the September update and the Store workaround first. Both are above, and both are better than this.
2. Open Settings, then Windows Update, then Update history.
3. Scroll to the bottom and select Uninstall updates.
4. Find the entry for KB5121003 and select Uninstall, then restart.
5. Install the September update (KB5124008, or KB5124012 on 26H1) as soon as you are able. That returns the security fixes and keeps the apps working.

Reported to work by 3 users; not confirmed by the vendor. 1 report it did not help.

Source: Microsoft Q&A, Microsoft Community Hub, Microsoft