Microsoft is warning customers about a new issue affecting the classic Outlook client on Windows, which may cause the app to crash immediately upon launch.
The Issue
- The bug impacts Microsoft 365 customers using classic Outlook.
- On affected systems, Outlook fails to start, displaying an error that Exchange login has failed.
- Logs often contain: “LID: 49586 - Authentication concurrency limit is reached.”
The Fix (Requires Microsoft)
Unlike most Outlook problems, this one cannot be fixed locally.
- Microsoft advises admins to open a support case in the Microsoft 365 Admin portal.
- Exchange Online support will then apply a service-side change to resolve the issue.
Workarounds
Until the bug is patched, Microsoft recommends:
- Switching to the new Outlook for Windows, or
- Using Outlook Web Access (OWA).
Background
This is the latest in a series of bugs impacting classic Outlook in 2025:
- CPU spikes while typing
- Broken drag-and-drop for calendar & email after Windows 24H2 updates
- Crashes when opening encrypted or external messages
✅ This case highlights the risks of relying on legacy clients. At Alcaeus Services, we help organizations transition to modern, secure platforms and implement resilient email strategies that minimize downtime when unexpected bugs like this occur.
