Outlook Won't Open? Fix "Cannot Open the Outlook Window" Error

Outlook throws up a dialog box instead of opening: "Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. The information store could not be opened." No inbox, no calendar, nothing. Restarting Outlook sometimes fixes it. Sometimes it doesn't.

I ran into this on a client machine running Outlook classic for Windows, build 2609 (Microsoft 365 Current Channel), connected to Exchange Online with Cached Exchange Mode on. It happened every time the PC rebooted, then worked fine for the rest of the day. This pattern points to a specific cause, and it's not the same fix as when the error shows up randomly during normal use. Below are both scenarios and how to fix each one.

What does "Cannot open the Outlook window" mean?

The error means Outlook is unable to read your mailbox data file, either the local OST cache or, less often, the connection to the Exchange server itself. Outlook builds your inbox from this file at startup. If the file is missing, locked by another process, or corrupted, Outlook has nothing to display and throws the dialog instead of opening.

This is different from a login error or a password prompt. You won't see a sign-in window. Outlook fails before it even gets there, which is why the usual password reset advice does not apply here.

Why does Outlook fail only on the first launch after a reboot?

This happens because a background service Outlook depends on, usually Windows Credential Manager or a OneDrive sync process, hasn't finished starting when Outlook tries to load. Outlook reads your saved credentials and OST file location from these services during startup.

If you reboot your PC and open Outlook within the first minute or two, you're racing Windows itself. Close the error dialog, wait 15 seconds, and open Outlook again. In most cases it opens clean the second time because Windows has caught up. If this happens every single reboot, set Outlook to start a minute after login instead of immediately, or build the habit of opening it after your other startup apps have settled.

How do you fix it when the error isn't tied to a reboot?

Work through these in order. Each one rules out a specific cause, and most people fix the problem within the first three steps.

Start Outlook in Safe Mode to test for a broken add-in

1. Close Outlook completely, including any background process in Task Manager.
2. Press Windows key + R, type outlook.exe /safe, and press Enter.
3. If Outlook opens normally in Safe Mode, an add-in is the cause.

To find the bad add-in, go to File > Options > Add-ins, select COM Add-ins in the Manage dropdown, click Go, and uncheck every add-in. Restart Outlook normally. If it opens, re-enable add-ins one at a time, restarting Outlook after each, until the error comes back. This add-in is the culprit. Disable it or check for an updated version.

Reset the Navigation Pane

A corrupted Navigation Pane file causes this exact error more often than people expect.

1. Close Outlook.
2. Press Windows key + R and run outlook.exe /resetnavpane.

This rebuilds your folder list from scratch without touching any actual email data. It's the single fastest fix and worth trying before anything more involved.

Rebuild the OST file

If Safe Mode and the Navigation Pane reset don't help, the OST file itself is likely damaged.

1. Close Outlook fully.
2. Press Windows key + R, type %LOCALAPPDATA%\Microsoft\Outlook, and press Enter.
3. Find the .ost file matching your email address (for example [email protected]).
4. Rename it to old-outlook.ost rather than deleting it, in case you need to roll back.
5. Open Outlook. It rebuilds a fresh OST file and re-downloads your mailbox from Exchange Online.

Expect this to take several minutes on a large mailbox, since Outlook is pulling everything down again from the server. Once it's done, confirm your folders and recent email load correctly, then delete the renamed old file.

Create a new Outlook profile

If the OST rebuild doesn't fix it, the profile configuration itself might be corrupted, separate from the data file.

1. Close Outlook.
2. Open Control Panel, search for Mail (Microsoft Outlook), and open it.
3. Click Show Profiles.
4. Click Add, name the new profile, and follow the setup wizard to add your Exchange Online account.
5. Set "Always use this profile" to your new profile, or set "Prompt for a profile to be used" if you want to choose at each launch.
6. Open Outlook using the new profile.

This creates a completely clean configuration. It's the most reliable fix when nothing else works, since it rules out every form of local corruption at once.

Does disabling Cached Exchange Mode fix it?

Turning off Cached Exchange Mode is a workaround, not a real fix. It forces Outlook to read your mailbox directly from the Exchange server instead of the local OST file, which sidesteps a corrupted cache. It also makes Outlook slower day to day and useless without an internet connection, so treat it as a temporary test rather than something to leave on.

To test it: File > Account Settings > Account Settings, select your account, click Change, uncheck Use Cached Exchange Mode, then restart Outlook. If Outlook opens fine with cached mode off, the OST file was the problem. Go back and rebuild it using the steps above rather than running without cache permanently.

If Outlook opens fine but keeps asking for your password on every launch, this is a separate issue. See Outlook Email Login Problems: How to Fix Sign-In Errors That Keep Coming Back for the fix.

When should you contact IT or Microsoft support instead of troubleshooting further?

Contact your IT admin or Microsoft support if the error keeps coming back after a fresh profile and OST rebuild, or if it started right after a specific Windows or Office update rolled out across multiple machines at once. A repeat failure after a full profile rebuild usually points to a server-side mailbox issue or a Group Policy setting pushed by your organization, and fixing either one requires help from IT, not more troubleshooting on your end.

Also reach out to IT directly if you're on a managed work account and don't have permission to run Control Panel's Mail setup or create a new profile. Forcing it without the right permissions leaves your account in a half-configured state, harder to fix afterward.

If Outlook opens but calendar invites crash it or shared mailboxes won't load, those are different bugs with their own fixes. See Classic Outlook Crashes When You Accept a Shared Calendar Invite and New Outlook Shared Mailbox Not Showing Emails.

Frequently Asked Questions

Will rebuilding my OST file delete my emails?

No. The OST file is only a local cache of your mailbox. Deleting it removes nothing from the server. Outlook rebuilds the file from your Exchange Online or Microsoft 365 mailbox the next time it opens, so every email, folder, and calendar item comes back. The only downside is a slower first sync, which takes a few minutes on a large mailbox.

Why does this error only happen on the first launch after I restart my PC?

Outlook is trying to open its data file before Windows has finished loading background services, most often the Credential Manager or a sync client like OneDrive. The profile isn't broken. Closing Outlook and opening it again a second time usually works, because by then Windows has finished starting everything Outlook depends on.