Redazione RHC : 2 October 2025 09:18
Microsoft has announced that it is investigating a significant issue with the traditional Outlook desktop client for the Windows operating system, which causes the application to crash upon startup.
When a user tries to open the classic version of Outlook for Windows, the problem occurs. Instead of loading the mailbox, the application displays an error message stating: “Cannot start Microsoft Outlook. Cannot open the Outlook window. Cannot open the folder set. The attempt to log on to Microsoft Exchange failed.”
To identify users potentially affected by this specific bug, Microsoft has provided IT administrators and technical teams with a precise diagnostic tool . By recording a Fiddler trace during the error occurrence, administrators can identify a well-defined exception.
As of September 26, 2025, Microsoft’s official status for the issue is “UNDER INVESTIGATION.” There is currently no direct fix that users or administrators can apply on their own.
The issue prevents users from accessing their mailboxes and displays a critical error message, disrupting daily workflows for the affected individuals and organizations.
Microsoft has noted that while there may be multiple causes for this generic error, recent support cases report a specific issue with user mailboxes attempting to access the service.
The presence of the following error in the log confirms the problem:
Microsoft.Exchange.RpcClientAccess.ServerTooBusyException: Client is being backed off --> Microsoft.Exchange.RpcClientAccess.ClientBackoffException: ErrorCode: ClientBackoff, LID: 49586 - Authentication concurrency limit is reached.This exception indicates that the connection is failing because the user’s client is exceeding the authentication concurrency limit set by the server, effectively being “backwarded” or throttled by Microsoft Exchange Online. This prevents the Outlook client from properly logging in and opening the user’s mailbox folders.
To resolve this issue, organizations must open a support case through the Microsoft 365 admin portal. The Exchange Online support team will then need to implement a backend change to mitigate the authentication issue for the affected mailboxes.
In the meantime, Microsoft has provided immediate workarounds to ensure users can continue to access their email. Users affected by this issue are advised to use Outlook Web Access (OWA ), the browser-based version of Outlook, or to switch to the new Outlook for Windows client.
These clients are not affected by the bug and represent a reliable alternative while a permanent fix for the classic desktop application is developed . Organizations are encouraged to communicate these workarounds to their users to minimize inconvenience.
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