Cannot open the Outlook window. The set of folers cannot be opened

Hi All,

My Outlook is reporting an error - "Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folers cannot be opened. The file c:\users\%username%\appdata\local\Microsoft\Outlook\XXXX.ost is not an Outlook data file (.ost).

How can I open OST file in Outlook.. Please suggest me?

  • Hi Feriha,

    Which Sophos product are you using?

  • If your Outlook reported an error ”cannot open the Outlook Windows, the set of folders cannot be opened” then there might number of possible reasons for Outlook to behave this way. You can implement the below-mentioned procedures to solve your issue.

    Procedure 1; Open Outlook in Safe Mode

    You can open Outlook in safe mode and delete the add-ins keys and it works fine.

    1. Press “Windows” Key + “R” (combine)

    2. Type “Outlook.exe /safe”(leave a space after Outlook) >> Click on “OK” button.

    But if it does not work for you then follow the procedure 2 as mentioned below.

    Procedure 2: Create a new Outlook Profile

    The below is a document about how to create a new Outlook profile. It has noticed that creating Outlook profile solves the different Outlook-related issue and you can also check its result.

    Procedure 3: Repair Corrupt OST File

    Sometimes Outlook fails to respond if there exist if there exists corruption issue in Outlook data file. You can run Inbox Repair Tool(Scanpst.exe) to repair corrupt OST file but it works only for the minor level of corruption. Hence if your OST file is majorly corrupted, you can use Microsoft OST to PST Converter Solution of any trusted brand to repair corrupt OST file.

    Hope it might help you!

    Let me know if you need any further assistance!

  • It's a commonly seen error in Microsoft Outlook. Sometimes this type of error appears when Outlook fails to recognize OST file due to some corruption causes and reported an error like; xxx.ost is not an offline folder file. You can undergo some manual methods for this.

    Step 1: Open the MS Outlook.

    Step 2: Create a new PST file in your Outlook Profile.

    Step 3: Then, drag all the mailbox folders which you want to move in the PST file.

    Note: You cannot drag Inbox, Contacts, etc. default folder into a PST file.  So, just copy the folder from OST to PST.

    However, if you are not able to resolve the error, you can go to a third-party OST to PST converter to fix this error.

  • OST file resides at local PC and sometimes we make a backup copy on Office 365 account to ensure safety of business emails. In this situation, you should use any powerful email security tool like Sophos Email Security.


    You can get its free demo from here:


    The above tool also works on Office 365 as an enhanced security gateway.

    Note: In case, Exchange OST is already damage and your organization want to export into PST or Office 365, then try some useful references mentioned below:

  • Hello, there are plenty of application but you should get the right solution as per need. We recommend you to not to purchase full version without using FREE demo version. It is right that there is no manual way to resolve the error so, you need to go with third party solution.

    As per my experience, I would like to suggest given below solution as it is tested.

  • Hi,

    If you are facing an error says - cannot start Microsoft Outlook. Cannot open the Outlook window, try an astounding third-party OST Recovery tool to get rid of any kind of OST related issues just in a short span of time. It recovers complete email messages, drafts, contacts, attachments, lists, journals, calendars, etc. and moves them to well-known Outlook PST and several other file formats which include MBOX, EML, MSG, DBX, etc.