Last Updated: June 23, 2023
Sometimes Outlook appears to be syncing the entire OST all or most times it starts up. Outlook will be stuck at the splash screen for a long time before eventually opening and operating normally.
Investigation of profile and OST file names will show new OST files are not being created (this would also appear in the Application Event Logs if it were the cause). Customers often report that if they cancel Outlook opening and try again, it will open up without delay. They may also note that if they have switched into Airplane mode, Outlook will sometimes refuse to start at all, giving this error:
“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.”
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. 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.
The Outlook Team is investigating a fix for this issue. We will update this topic when the fix is available.
This issue happens because Outlook is having trouble determining the "default" state of the cache, with Outlook and non-Office MAPI apps like MFCMAPI coming to different conclusions.
One workaround is to use the group policy setting in the article How to enable and disable the Outlook calendar sharing updates.
This setting, when set to 1, will force REST on and avoid the issue:
Value: 1 if Enabled, 0 if Disabled
This setting is also available in non-policy form:
Another solution is to explicitly specify the REST state in the Outlook account settings. This is done by toggling "Shared Calendar Improvements" off and on:
File/Account Settings/Account Settings
On the Email Accounts dialog, click on the Name of the Exchange account and click Change.
Note that "Turn on shared calendar improvements" is on.
Toggle it off, then immediately toggle it back on.
Outlook may warn about needing to restart. It doesn't, but you can if you want to.
Click The X to close the Exchange Account Settings dialog.
Here's a picture of the dialog:
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