Microsoft has recently fixed the problem that the Outlook desktop client cannot access group mailboxes and calendars for Outlook for Microsoft 365 users.
The Outlook versions in question include the version released to the Current Channel on March 28, 2023, and Outlook Desktop Version 2304 Build 16327.20214.
The attached questions are as follows:
Can't add or access group mailboxes Can't add or access group calendars Cannot add, delete or edit group members Unable to mark email or item as read Unable to flag, file or move messages or items to other folders
Microsoft describes the cause of the issue under EX540503 in the Microsoft 365 Admin Center: “A recent standard servicing update inadvertently contained an authentication code regression that prevented some users from accessing or performing various Microsoft 365 Group Operations”.
Affected Outlook users can go to File > Office Account > Update Options and click Update Now to deploy.