Microsoft is working to address a known issue affecting Outlook for Microsoft 365 customers, causing slow starts and freezes as if Offline Outlook Data Files (OST) are being synced right after launch.
However, no new OST files are being created, and no new entries associated with this behavior are added to the Application event log, according to Redmond's investigation.
Many customers impacted by this issue have also reported that Outlook will open promptly, without delay, if they hit cancel after opening the application.
Microsoft says that affected customers may also see that the application will refuse to start on systems where Airplane mode is enabled.
In such cases, users will see errors saying, "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 Outlook freezes and very slow starts happen during cache re-priming, which occurs when creating new profiles or turning off shared calendar improvements.
"One known cause of this symptom is cache re-priming. This is expected if the user has created a new profile, or manually toggled from REST to MAPI by disabling Shared Calendar Improvements," Microsoft says.
"However, due to endemic problems in reading service enabled feature's status in non-Office MAPI based applications, running such apps can force this toggle unexpectedly."
Workaround availableThe Outlook team is currently investigating and working on fixing this problem caused by the app experiencing issues when attempting to determine what Microsoft described as the "default" state of the cache.
This happens because non-Office MAPI apps like MFCMAPI and Outlook will arrive at different conclusions, causing unexpected hangs and slow starts.
A support article published today says that affected Outlook users can do so by toggling on the "Shared Calendar Improvements" option in the Exchange Account Settings dialog after clicking More Settings and selecting the "Advanced" tab (the change will be applied after restarting Outlook).
Enabling Outlook shared calendar improvements (Microsoft)IT admins can also use a group policy setting to enable Outlook REST calendar-sharing updates to work around the issue on impacted systems.
In April, Microsoft fixed another issue blocking Outlook for Microsoft 365 customers from accessing their emails and calendars.
Redmond also shared temporary fixes for login issues in October 2022 and a bug causing Outlook to freeze and crash right after launch in August 2022.
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