Classic Outlook crashing the moment someone accepts a shared calendar invite? Editing one occurrence of a recurring meeting and watching the change hit every instance? You've run into the shared calendar improvements rollout, and Microsoft has both issues on its known issues list right now.

What's Breaking

Microsoft is moving shared calendars in classic Outlook from the old MAPI sync to a newer REST-based sync. The transition is causing a cluster of confirmed issues.

Crash on accepting invites. Classic Outlook may crash when accepting a shared calendar invitation. Microsoft listed this in July 2026 and still has it marked as investigating.

Recurring meeting edits go too far. In Microsoft 365 Group calendars, modifying a single instance of a recurring meeting updates every instance.

Attachments reappear after deletion. Deleted calendar attachments come back. Microsoft's workaround: give the sync about a minute to finish before moving on, or keep files in SharePoint or OneDrive instead of attaching them.

Delegate quirks. Delegates miss attachment updates and can see sync errors, and delegates with limited access can't send meetings with sensitivity labels applied.

Quick Mitigation

While Microsoft investigates the crash, you can flip an affected account back to the old sync path. In classic Outlook go to File, Account Settings, Account Settings, double-click the account, click More Settings, and open the Advanced tab.

Uncheck Turn on shared calendar improvements, then restart Outlook. This reverts shared calendars to the older MAPI behavior. Treat it as temporary: the REST path is where Microsoft is headed, so re-enable it once the crash is fixed.

If That Didn't Work

Make sure Office is current, since fixes for these ship through updates. In any Office app go to File, Account, Update Options, Update Now.

If the crash persists on a current build with improvements turned off, create a new Outlook profile before blaming the machine. Corrupt profiles produce identical symptoms.

Track the status yourself on Microsoft's official pages. Both are updated as issues move from investigating to fixed.

Known issues in classic Outlook for Windows

Known issues with shared calendar improvements

Fix Classic Outlook MSVCP140.dll Crashes

Fix Teams Meeting Button Missing from Outlook

Something Went Wrong in Outlook, Teams, and Microsoft 365

Shared mailboxes and calendars acting up across your team? Contact Rain City Techworks.