Teams and Outlook Went Dark on ARM Windows PCs
Since Windows security updates released on or after August 11, 2026 (KB5121003), Microsoft Teams and the new Outlook for Windows failed to launch — or closed immediately after opening — on ARM-based Windows 11 24H2+ devices, including the Surface Pro 11 and Surface Laptop 7. The problem mainly hit new or freshly imaged machines that hadn't yet pulled the latest Microsoft Store app updates, making it an especially unwelcome surprise for IT teams provisioning new Copilot+ hardware.
Microsoft never published a detailed root-cause explanation, but confirmed the failures were connected to the interaction between the August security update and Store-delivered app dependencies. As a stopgap, the company pointed affected users toward updating the Auto Super Resolution Package from the Microsoft Store, which resolved the issue for some devices ahead of a permanent fix.
The Fix
Microsoft resolved the issue with the September 2026 Patch Tuesday cumulative update, KB5124012. The company's guidance is straightforward: install the latest cumulative update and let the Microsoft Store apply pending app updates before relying on Teams or Outlook on affected hardware.
Why This Matters
A month-long window where two of the most heavily used business communication apps wouldn't reliably launch is a reminder that ARM Windows compatibility issues can still slip through in routine security updates — ARM hardware has a thinner real-world testing history than x86, and app-plus-OS interaction bugs like this one are easy to miss in pre-release validation.
Practical takeaways for IT teams running or piloting ARM/Copilot+ fleets:
- Verify KB5124012 is installed across ARM Windows 11 devices before treating the Teams/Outlook issue as resolved.
- Force Microsoft Store updates on freshly imaged ARM machines before handing them to end users — the bug disproportionately hit devices that hadn't yet pulled Store updates.
- Validate critical line-of-business and communication apps after every Patch Tuesday, particularly on newer or less-common hardware architectures, rather than assuming parity with x86 behavior.
- Keep an eye out for any follow-up advisory from Microsoft detailing the actual root cause, since the company has not published one to date.