Microsoft Teams Desktop App Failing to Launch on Windows

Microsoft is investigating an ongoing problem that causes the Teams desktop app to load slowly or fail to open entirely on some Windows machines. The issue is tracked as TM1466820 and remains unresolved.

To be clear about what this is: there is no indication this is a security incident. It is a reliability problem. But it is disrupting a tool most organisations depend on for daily communication, so it belongs on IT teams' radar.

Affected users hit the problem on the first launch of Teams on a Windows device. Some cannot load the client at all. Others wait up to two minutes before the app becomes usable.

Microsoft has scoped the impact to "some users" within affected organisations, particularly those opening Teams to check new messages, and says impact varies between tenants. Not every organisation will see it, and within affected organisations not every user will.

In its most recent update, posted on the morning of September 4, Microsoft said its analysis of service logs and telemetry has so far been inconclusive. Engineers have not identified a root cause.

The company is now contacting a subset of affected users to collect client-side logs from their devices — which suggests a targeted investigation rather than a fix arriving imminently. No resolution timeline has been given.

Teams on the web and the Teams mobile app are both reportedly unaffected. Neither fixes the underlying problem, but both let people keep working.

Tell staff about the official workarounds before they go looking for their own. This is the one genuine security angle. When a familiar tool breaks and no one explains why, people search for fixes, download things they shouldn't, and become receptive to messages offering help. Outages are a reliable setup for phishing. A short internal notice — "Teams desktop has a known issue, use teams.microsoft.com in your browser" — removes that opening.

Monitor the service health dashboard in the Microsoft 365 admin center for updates on TM1466820 rather than relying on user reports.

Don't start reimaging machines. With impact varying by tenant and no root cause identified, local troubleshooting is likely wasted effort.

This is not the first Teams launch problem this year. A regression in the client's build-caching system caused similar failures earlier in 2026, which Microsoft fixed by rolling back a faulty service update. Separately, Windows 11 users on ARM-based devices, including some Surface models, hit Teams and Outlook launch failures tied to an August security update, since mitigated through a Microsoft Store update.

Sources: Cyber Security News · Microsoft 365 Service Health