2 min

Outlook users have reported sluggish starts and freezing. It is giving them the impression that Offline Outlook Data Files (OST) are syncing immediately after launching the application. However, Microsoft’s investigation reveals that no new OST files are being created, and there are no corresponding entries in the Application event log.

Curiously, some affected customers have discovered a workaround. If they hit the cancel button after opening the application, Outlook promptly opens without delay. Microsoft also highlights that users may encounter problems launching Outlook on systems with Airplane mode enabled.

In such cases, error messages like “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” may appear.

The root cause of these freezes and slow starts in Outlook lies in cache re-priming

Cache re-priming occurs during the creation of new profiles or when users manually switch from REST to MAPI by disabling Shared Calendar Improvements.

However, non-Office MAPI-based applications can unexpectedly force this toggle due to difficulties in reading the feature’s status. To address the issue, the Outlook team is diligently investigating the problem related to determining the “default” state of the cache.

The disparity in conclusions reached by non-Office MAPI apps like MFCMAPI and Outlook leads to unforeseen hangs and slow starts.

What to do

Microsoft advises affected users to toggle on the “Shared Calendar Improvements” option in the Exchange Account Settings dialog, accessible by clicking More Settings and selecting the “Advanced” tab.

IT administrators can also implement a group policy setting to enable Outlook REST calendar-sharing updates, providing a workaround for impacted systems.

This isn’t the first time Microsoft has grappled with Outlook issues. In April, the company resolved an issue that blocked Microsoft 365 customers from accessing their emails and calendars. Last year, they offered temporary fixes for login problems in October and a bug causing Outlook to freeze and crash immediately after launch in August.

Also read: Web-based Outlook to replace mail, calendar apps on Windows 11