Microsoft is offering some workarounds for a bug in Outlook for Windows which leaves it hanging on the opening screen and very slowly opening properly.
According to Microsoft the bug is in the Outlook software part of Microsoft 365 for Windows. We suspect a similar problem might also be in Outlook 2021 for Windows since the two products share a lot of code.
The bug appears when starting Outlook 365 for Windows. It hangs on the opening splash screen which looks like this:
Outlook appears to be syncing with the main mailbox but isn’t.
Cancelling Outlook’s startup (using the X button at top right) and restarting seems to fix the problem in many cases.
However, if the computer is offline (e.g. Windows ‘Airplane’ mode) a nasty error might appear:
“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.”
What’s happening?
The smarties at Microsoft think the problem is with the way Outlook caches data. Outlook can get confused when the cache option changes.
There are two caching options called REST and MAPI. REST is preferred these days but some apps might force Outlook to use MAPI.
Three Workarounds
If you strike this problem there are three workarounds. All of them amount to the same thing; forcing Outlook to use REST.
The easiest way to do that is in Outlook mailbox settings. File| Account Settings | Account Settings | choose a mail store | Change | More Settings | Advanced
Turn on shared calendar improvements – should be ON. The calendar improvements require REST so enabling this option should do the trick. According to Microsoft, a restart of Outlook isn’t necessary.
If Outlook isn’t or can’t start – the same option is available from Control Panel | Mail (Microsoft Outlook) | Email Accounts then choose an account and follow the same steps as above.
However, that option might not be enough. When we tested that choice, our Outlook calendars (hosted on Microsoft 365) stubbornly remained as MAPI. Check the calendar folder Properties | General | Type will have either MAPI or REST in brackets.
Registry
The other workarounds also force REST in Outlook from either the registry or a policy.
On individual machines:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\Calendar
As part of a policy use:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\Options\Calendar
Look for or, more likely make a new key:
Type: DWORD
Key: RestUpdatesForCalendar
Value: 1 to enable or 0 to disable.
Force REST by using the value 1.
