Folders in Outlook Classic empty after update

This usually happens when Outlook’s local data file (PST or OST) isn’t syncing correctly after a Windows update. The update you mentioned (KB5066835) has caused similar issues for other users. To fix it, try checking your Outlook account settings to make sure the correct data file is set as default, then reset your folder view under the View tab. If that doesn’t help, you can recreate your Outlook profile or delete and rebuild the local cache (OST file), which forces Outlook to re-sync all folders from the server. Since your emails appear in Outlook Modern and on the web, this is almost certainly a sync or profile issue, not data loss. Hopefully, after reconnecting or rebuilding the data file, your folders and emails will show up again locally.

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