Software issue · Evidence-backed record

Teams / new Outlook fail to launch on ARM after KB5121003

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Direct answer

Microsoft confirmed launch failures or unexpected exits in Teams and the new Outlook on ARM devices after the August 2026 security update. New or freshly imaged PCs without Store updates are more likely to be affected. Windows 11 24H2/25H2 updates from KB5124008 onward resolve this documented issue.

Resolved Microsoft [1] · Microsoft [2] · Microsoft [3]

Check affected environments Find a remedy for your version

Status: Resolved FACT
Affected: Windows 11 26H1; Windows 11 25H2; Windows 11 24H2
First observed: Unknown
Last verified: Oct 11, 2026, 2:43 PM UTC
Confidence: Vendor confirmed
Severity: Unknown

TraceCheck interpretation

Does this explain your situation?

Use these checks to interpret the source. This is a reading guide, not a device test or a vendor diagnosis.

Check architecture and application
This record requires an ARM environment and Teams or the new Outlook. A similar crash on x64 or classic Outlook needs separate evidence. Fresh installation history strengthens the match but is not a requirement for every affected device.
Source: Microsoft
Separate the origin and the resolution
For 24H2/25H2, KB5121003 identifies the originating release and KB5124008 the fixing release. The 26H1 record instead names KB5121000 and KB5124012. Apply the update for your own Windows version.
Source: Microsoft
Use the Store workaround only where relevant
Microsoft offers the Store-package workaround for affected devices still on updates predating September 8. It is unnecessary after the documented Windows fix. Check the installed Windows release before adding that extra step.
Source: Microsoft

Symptoms

Teams or the new Outlook may fail to open or close unexpectedly on an ARM device. Source: Microsoft
Classic Outlook, Word and Excel are not known to be affected by this specific issue. Source: Microsoft

Affected environments

Evidence-backed affected targets. An omitted dimension has no recorded conclusion.
Entity	Version	Originating update	Architecture	Software	Evidence
Windows 11	26H1	Unknown	ARM	Microsoft Teams; new Outlook for Windows	Source: Microsoft
Windows 11	25H2	KB5121003	ARM	Microsoft Teams; new Outlook for Windows	Microsoft [1] · Microsoft [2]
Windows 11	24H2	KB5121003	ARM	Microsoft Teams; new Outlook for Windows	Microsoft [1] · Microsoft [2]

What caused it?

Root cause has not been confirmed.

Fixes and workarounds

A version match identifies recorded guidance, not a diagnosis or a guarantee. Check the symptom, device and application requirements in each source before proceeding.

Install the applicable Windows update for ARM app launches

FACT Official fix · vendor fix

Microsoft records the resolution in KB5124008 and later Windows 11 24H2/25H2 updates. The linked package is historical, not necessarily the latest applicable update. This package scope excludes 26H1.

Documented symptom resolution

Applies to: Windows 11 24H2, Windows 11 25H2.

Risk: medium (TraceCheck editorial assessment, not a vendor success rate)

Verification: Oct 11, 2026, 2:36 PM UTC

Fixing update: KB5124008 · Source: Microsoft

Read Microsoft's instructions ↗

Store-package workaround for affected pre-September 8 devices

FACT Official workaround · workaround

For affected devices on a Windows update predating September 8, open Microsoft Store, select Downloads, check for updates, and install Auto Super Resolution Package version 1.0.19.0 or later. Microsoft says this workaround is unnecessary after the Windows resolution.

Documented symptom resolution

Applies to: Windows 11 24H2, Windows 11 25H2, Windows 11 26H1.

Risk: low (TraceCheck editorial assessment, not a vendor success rate)

Verification: Oct 11, 2026, 2:36 PM UTC

Source: Microsoft

Read Microsoft's instructions ↗

Current status

Resolved This is the recorded state as of the last verification, not a live device diagnosis. Microsoft [1] · Microsoft [2] · Microsoft [3]

Vendor acknowledgement / confirmation: 2026-09-02 01:46:00 UTC.

Resolution recorded: 2026-09-08 17:00:00 UTC.

Updates linked to documented corrections: KB5124008

For the recorded vendor guidance, see ARM Teams / Outlook issue — KB5121003 ARM Teams / Outlook — Windows 11 26H1 resolved issues ARM application launch dates — Windows 11 24H2 release health.

Evidence

ARM Teams / Outlook issue — KB5121003
Microsoft · Vendor documentation
Supports: ARM application launch failure and Windows 11 24H2/25H2 originating update; vendor notes point to the September resolution.
Retrieved Oct 11, 2026, 2:36 PM UTC
ARM Teams / Outlook — Windows 11 26H1 resolved issues
Microsoft · Vendor documentation
Supports: Affected ARM apps/platforms, acknowledgement/resolution dates, 26H1 KB differences and conditional Store-package workaround.
Retrieved Oct 11, 2026, 2:36 PM UTC
ARM application launch dates — Windows 11 24H2 release health
Microsoft · Vendor documentation
Supports: Vendor acknowledgement and resolution dates, with KB5124008 as the 24H2/25H2 fix.
Retrieved Oct 11, 2026, 2:36 PM UTC

Change history

2026-09-08 17:00:00 UTC
Confirmed → Resolved
Microsoft records the September 8 Windows update resolution.
Source: Microsoft
2026-09-02 01:46:00 UTC
Confirmed
Microsoft opened the ARM application launch record.
Source: Microsoft

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