Outlook "Cannot Expand the Folder" Error on a Public or Shared Folder? Here's the Fix

Classic Outlook pops a Microsoft Exchange dialog and then errors with “Cannot expand the folder. The set of folders cannot be opened” when you click a public folder (and sometimes a shared or archive folder).

Why: Two distinct causes share this wording. Often the message continues with “the file …\Outlook.ost is in use and cannot be accessed” — another process is locking your data file. On Exchange public folders, it’s instead that the AutoDiscover service can’t resolve the address stamped on the public folder mailbox, so Outlook never finds the folder store.

Fix 1: Release the locked data file (.ost)

Start here if the error mentions the file is in use.

1. Close Outlook completely.
2. Press Ctrl+Alt+Delete → Task Manager → Processes tab.
3. End any Outlook.exe and Winword.exe (Word) entries still running — Word holds the data file open when it’s your email editor.
4. Reopen Outlook and expand the folder again.

If that clears it, the file was simply locked by a leftover process.

Fix 2: Rebuild the Outlook profile

A corrupted profile is the most common end-user cause, and rebuilding is Microsoft’s standard step.

1. Close Outlook. Open Control Panel → Mail (Microsoft Outlook) → Show Profiles.
2. Click Add, create a fresh profile, add your account, and set Always use this profile.
3. Open Outlook on the new profile and try the folder. A clean profile re-runs AutoDiscover and re-downloads the folder list.

Fix 3: AutoDiscover (admin) for Exchange public folders

If this hits multiple users on public folders specifically, the fix is server-side, per Microsoft.

• The cause is that AutoDiscover can’t discover the SMTP address stamped on the public folder mailbox.
• Have your Exchange admin either create the AutoDiscover DNS records for that email domain, or change the public folder mailbox’s email address to a domain that already has published AutoDiscover records.
• Individual users can’t fix this one — it has to be done on the Exchange/DNS side.

FAQ

Is this the same as “Cannot start Microsoft Outlook”? No. That start-up error blocks Outlook entirely — see Cannot start Outlook / set of folders cannot be opened. This one lets Outlook open but blocks one folder.

It happens only on a shared mailbox. Re-add the mailbox cleanly — see Add a shared mailbox in Outlook — and if it persists, use Fix 2 to rebuild the profile.

Sources: Microsoft Learn — Outlook users can’t access public folders (Exchange)