Microsoft suggests new bug breaks Windows Outlook so bad there's no way to 'fix' it

Last month, Microsoft announced that it is killing the lightweight version of the Outlook app, known as Outlook Lite. The company has its reason for the retirement and has recommended an alternative to users.

Aside from the lite version, Microsoft is also very determined to move on from Classic Outlook and tries various ways to get users onto the New Outlook for Windows app. For example, last month it revealed how it will "automatically and seamlessly set up users’ classic Outlook accounts and settings in the new Outlook for Windows" as the company confirmed this was its way to not "force" users to use the new app.

Besides such moves, the tech giant has also been quite verbal about its admiration of the new app and has explained more than once why users should switch to New Outlook already.

With all the push for the new app, it seems Microsoft may be losing focus on development and QA testing of the classic app as clearly it is not a priority any more. Over the last several months, there have been several confirmed bugs in Classic Outlook.

For example, in April, users found that downloading the app was broken, and later in June, after a major Calendar feature upgrade, the classic app got hit with several different bugs, which took months to fix; and once again in the same month, the ability to send emails, which is the basic functionality of a mail app, was impacted too, due to yet another underlying issue.

A new bug has now hit classic Outlook as Microsoft confirmed that the application won't open with an error message that says "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."

In the issue description, Microsoft has added more details about it and also how admins and users can confirm the bug with the help of Fiddler trace. If you are not familiar, a Fiddler trace helps to inspect and monitor traffic for the purpose of debugging a web issue. The company explains:

This error message can occur for different reasons. Not every instance of this error is the same issue but recent support cases around this have been for user mailboxes.

 

To confirm if this issue applies, capture a Fiddler trace when you reproduce the problem and then search the trace for this error:

 

Microsoft.Exchange.RpcClientAccess.ServerTooBusyException: Client is being backed off
---> Microsoft.Exchange.RpcClientAccess.ClientBackoffException: ErrorCode: ClientBackoff, LID: 49586 - Authentication concurrency limit is reached.
--- End of inner exception stack trace ---

Microsoft says that it is investigating this problem trying to get to the root of it although right now the company notes that there is no way to fix it, and that "the only way to address this issue is to open a support case from the Microsoft 365 Admin portal" to "mitigate" it.

Thankfully there is a workaround and according to Microsoft it is very simple; you can use the New Outlook for Windows app or the Outlook Web Access (OWA). You can find the issue on this support article here on Microsoft's official website.

Source

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Posted Friday 3 October 2025 at 5:10 pm AEST (my time).

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