Microsoft 365 Outlook Bug Leaves Businesses Without Email Access
Millions of businesses worldwide, particularly those using Microsoft 365, are grappling with a critical bug in the classic Outlook desktop application for Windows. This issue is preventing users from accessing their emails, especially in medium to large enterprises with complex IT environments and shared mailboxes.
The bug, identified as a 'ClientBackoffException', occurs when the permitted number of simultaneous authentications is exceeded. This causes the Exchange server to block connection attempts, leading to the error message: 'Microsoft Outlook cannot be started. The Outlook window cannot be opened. The folder set cannot be opened. The attempt to log in to Microsoft 365 has failed.'
In the meantime, Microsoft advises affected users to switch to Outlook Web Access (OWA) or the new Outlook for Windows as a temporary workaround. To check if your system is affected, Microsoft recommends analyzing a Fiddler trace. Affected companies should open a support case in the Microsoft 365 admin portal to initiate a service change for a fix.
Microsoft is currently investigating the issue and working towards a resolution. Until then, users are advised to use the recommended temporary workarounds to access their emails. The cause of the error, a 'ClientBackoffException', highlights the need for careful management of simultaneous authentications to prevent such blocks in the future.
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