There are not exactly a few Office users who face the problem that cannot access Outlook OST, must connect to Microsoft Exchange. It’s an error message that appears out of nowhere. Although, luckily, solving it is something too simple that we will teach you a little further down.

We are trying to check our email inbox from Outlook as usual and all of a sudden we get an error saying something like this:

“Cannot start Microsoft Outlook. The Outlook window cannot be opened. Unable to open set of folders. The file .ost cannot be accessed. You must connect to Microsoft Exchange at least once before you can use your Outlook data file (.ost).”

Definitely not what we expected to find. But don’t worry, we teach you how to skip this problem and leave it behind so you can continue using Outlook as usual.

Cannot access Outlook OST, must connect to Microsoft Exchange

Edit the registry

The Registry Editor in Windows allows us to do several very interesting things in Windows. Although we always recommend making a backup of it before starting to modify it.

After the user changes the Microsoft account password, Outlook prompts us to connect to Microsoft Exchange to be able to access the Outlook data file, but it does not show a window where to type the new password. In order to solve this problem, we will have to force Outlook to allow us to enter the username and password.

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For this we are going to use the Registry Editor, we start by pressing Windows + R, we will type regedit and press Enter. Once we are in the Registry Editor, we are going to have to navigate to the following location via the left sidebar:

HKEY_CURRENT_USERSoftwareMicrosoftOffice16.0CommonIdentity

We will look for EnableADAL on the right side. In case we do not find the value, we will have to right-click on an empty space and select New > DWORD (32-bit) Value. We will rename the value to EnableADAL, double-click it, and set the value data to 0.

Next, we look for DisableADALatopWAMOverride, in case we can’t find it, we’ll create a new value and name it DisableADALatopWAMOverride. The value data has to be 1.

To finish, we close the Registry Editor and proceed to restart the computer. Once it starts, we open Outlook and check if the problem persists.

Clear Outlook credentials

Something else we can try is to remove the credentials with the Windows Credential Manager and then add them again. To do this, we will have to launch Credential Manager from the Start menu and Outlook credentials. After clearing the credentials, we will proceed to start Outlook to log in again.

  • From the start menu, we type Credential Manager.
  • We look for the Outlook credentials, we will know that they are from Outlook because they will have the word Outlook.
  • We expand it and click Delete.

Repair Outlook

Another option we have is to repair Outlook. Best of all, Windows has a tool that takes care of doing this automatically. So we won’t have to intervene in the process.

  • Press the Windows + i key combination to access Settings.
  • Now we go to Applications > Applications and features.
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Here we will have to search for the application and select Advanced Options in Windows 10. In the case of Windows 11 we will have to click on the three vertical points of the Outlook app and we will be able to see Advanced Options.

To finish, we will click on Repair. Something that we must consider is that the process can be a bit slow, so we will have to be patient.

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