Outlook 365 Group Calendars disappear

Microsoft published this advisory (EX540503) yesterday:

Title: Some users may be unable to view or access group calendars and email messages in the Outlook desktop client

User impact: Users may be unable to view or access Microsoft 365 group calendars and email messages in the Outlook desktop client.

More info: Affected users are part of a Microsoft 365 group and are on versions 16.0.16222.10000 or higher, including the latest February release channel. Some affected users may also notice that impact self-resolves after some time, although impact may also reoccur at a later time. These users may also be unable to perform one or more of the following actions in the Outlook desktop client:

- Add or access the group mailbox
- Add or access the group calendar
- Add, remove, or edit group members
- Changes to mail items, such as marking as read, archiving, or moving email to other folders.

Current status: We've identified that a recent service update is incorrectly triggering an indexer to remove specific values within the Outlook desktop client for some users, resulting in impact. We've developed a fix to repair this issue and are validating it within our internal test environments to ensure all affected scenarios are accounted for before proceeding.

Scope of impact: Impact is limited to a small number of users on versions 16.0.16222.10000 or higher, and are attempting to access or perform various Microsoft 365 group related actions in the Outlook desktop client.

Start time: Tuesday, March 28, 2023, 6:38 PM (11:38 PM UTC)

Root cause: A recent service update is incorrectly triggering an indexer to delete specific values within the Outlook desktop client for some users, resulting in impact.

Next update by: Wednesday, April 19, 2023, 2:00 PM (7:00 PM UTC)