Microsoft Outlook Bug on Windows Devices Results in Repeated Email Crashes – Computer Emergency Response Team (Mauritius)

Microsoft is currently investigating a significant bug affecting classic Outlook for Windows that prevents users from accessing their email accounts.

The issue manifests as a persistent error message stating “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.”

The error appears when users attempt to launch the traditional Outlook desktop application on Windows devices.

According to Microsoft’s official documentation updated on September 26, 2025, this particular error can occur for various reasons, but recent support cases have primarily involved user mailboxes experiencing authentication failures.

Microsoft has identified specific technical indicators to confirm whether users are experiencing this particular issue.

Technical diagnostics reveal the error stems from a “ClientBackoffException” where the authentication concurrency limit is reached, causing the Exchange server to reject connection attempts.