MS Outlook is a prominent email client which provides facility to the users to work offline with OST file, even if it is not in synchronization to MS Exchange Server. The entire data including emails, calendars, journals, contacts, drafts, etc. are accessible in offline mode. Further, it gets synchronized with the MS Exchange Server when the Internet is available. The default location of the OST file is: drive:\Users\\AppData\Local\Microsoft\Outlook.

The OST file also is known as Offline Storage Table which helps users to store their mailbox and other data locally in the client machine in the form of OST file. However, sometimes MS Outlook users come across an error says “Default Outlook Profile .ost cannot be opened,” which indicates that the MS Outlook OST file can not open.
Although, there are many similar errors are also faced by users. Few of them are here:

  • Microsoft Outlook is not responding.
  • Default Outlook profile OST is in use and cannot be accessed
  • The Outlook.ost file is in use and cannot be accessed
  • Microsoft OST file is in use by another application
  • The set of folders cannot be displayed

Some possible causes for getting Default Outlook Profile .ost cannot be opened:
There are various reasons which may cause above mentioned error. Few of them are listed below:

  • Virus/Malware attacks
  • Poor/slow Internet connectivity
  • Trying to access same Offline OST file by many users at the same time
  • Other programs are running with the Outlook.
  • Due to the different OST file location
  • Incorrect or improper installation of Outlook

Note: Take a backup copy of Outlook OST files before initiating manual methods.

Manual methods to fix Default Outlook Profile .ost cannot be opened error:
To correct the above-stated shortcomings, users have to perform some manual process using below-mentioned methods.

Method 1: Close Outlook program and kill entire processes related to Outlook

  1. Make sure Outlook program is close.
  2. Just right-click on the Taskbar menu below and click on Start Task Manager to open Windows Task Manager. Or (Use keyboard shortcut by pressing Ctrl+Shift+Esc altogether).
  3. Now move on to the Processes tab.
  4. Into the Processes tab, look for the processes named with Communicator, ucmapi, Outlook or Lync and if you find any of these, select and click on End Process button to kill the processes.

Method 2: Disable Cached Exchange Mode

  1. Close MS Outlook
  2. Click on Start button and go to Control Panel.
  3. In the Control Panel, click on Mail option and then select Email Accounts.
  4. Under the Email Accounts, click on Email tab.
  5. Now click on Exchange account, and click on Change to process.
  6. Now uncheck the Use Cached Exchange Mode under offline settings, and hit Next to finish the process.

Disadvantages of Manual methods:
Although, manual processes are free to fix the above-mentioned error. But they have some limitations, as manual methods are less reliable, time taking and tedious tasks to perform. Non-technical users also face difficulty to perform manual processes.

What to do next, If nothing works?
If both the above methods don’t work to fix the “Default Outlook Profile .ost cannot be opened,” you can opt for a ScanPST.exe tool to correct this error. Although, ScanPST.exe only works in the case of minor OST file corruption. Hence, it is always recommended to use third party OST to PST Converter tool to recover and convert severely corrupted OST file and its associated data to PST.

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