Microsoft Outlook Bug on Windows Devices Results in Repeated Email Crashes

Microsoft is investigating a bug affecting classic Outlook for Windows that prevents users from accessing their email accounts, with the issue manifesting as a persistent error message: “Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened.

The attempt to log on to Microsoft Exchange has failed.” According to Microsoft’s official documentation, updated on 26 September 2025, the error can occur for various reasons, but recent support cases have primarily involved authentication failures. Technical diagnostics identify a ClientBackoffException where the authentication concurrency limit is reached, causing the Exchange server to reject connections.

The problem significantly disrupts business productivity, limiting access to emails, calendars, and contacts through the desktop client. While Microsoft classifies the issue as under investigation, the only definitive option currently is to open a support case through the Microsoft 365 Admin portal to request service‑level changes, with organisations advised to consider temporarily migrating affected users to Outlook Web Access or to the newer Outlook for Windows as workarounds.