New Outlook Reset After Windows Update: Accounts Gone, Cause Found in the Log

After restarting for the Windows updates in November 2026, the new Outlook for Windows started on one of my devices as if it had just been installed. The business account configured as the primary account was missing, along with the other accounts, signatures, and personal settings. There was no error message; Outlook went straight to the account setup wizard.

The cause is documented in Outlook’s local log: the app deleted its own local data because a newly enabled check assessed the state as “reset required.” There is no indication of interference by Windows Update, a user, or an attacker.

Timeline

The timestamps in the Olk log and in log file names are in UTC. They have been converted to local time (CET) in the table.

The log entries

The first lines of the new session show the check and its result:

Checking flight enabled status  flightName nh-userDataSentinel  isEnabled true
Reset sentinel check completed.  installState ResetRequired
  settingsFound true  resetSentinelFlagFound true
Reset sentinel check triggered user data reset.
Prompted for clear local state  clearLocalStateName Silent

This is followed by the deletion operations, each with succeeded true:

A few lines later, it says Last update date file missing, creating it. Barring clearing local state, this is likely a fresh install. From that point onward, Outlook behaves like a new installation.

Why the check was triggered

nh-userDataSentinel is a Flight, meaning a feature that Microsoft enables server-side for individual installations. In all older logs on the device, this Flight is listed as Flight status not set; using default (false), most recently at 13:23 UTC on the same day. At startup at 17:41 UTC, it was active for the first time.

The check looks for a marker file in the app’s package folder:

%LOCALAPPDATA%\Packages\Microsoft.OutlookForWindows_8wekyb3d8bbwe\LocalState\OlkSentinel

If Outlook finds user settings in the Olk folder (settingsFound true) but no matching marker in the package folder, it treats this as an indication that the app was reset or reinstalled and deletes the user data. On the affected device, OlkSentinel was newly created at 18:41:56, while the other files in LocalState date from September 2026. The package folder itself was therefore not reset; the marker file simply did not exist previously because the check was new. On the first start with the Flight enabled, the result was therefore a reset even though none was necessary.

According to the logs, the Windows Update restart was only the timing, not the cause: the Flight is evaluated at startup, and the first start after the update was the first with the Flight enabled. The app version (1.2026.1026.100) was the same before and after the restart.

Verifying the case on your own devices

The following query searches all Olk logs for entries from the check:

$logs = "$env:LOCALAPPDATA\Microsoft\Olk\logs"
Get-ChildItem $logs -Filter 'olk_*.log' |
  Select-String -Pattern 'Reset sentinel|Delete directory tree' |
  Select-Object Filename, LineNumber, Line

Outlook packages older sessions into ZIP archives in the same folder. If the reset occurred a few days ago, the entry may only be in the archive and must be extracted first.

Creation times provide a second indication: if EBWebView and UserSettings.json in the Olk folder, as well as OlkSentinel in the package folder, have the same timestamp, the reset occurred at that time.

$olk = "$env:LOCALAPPDATA\Microsoft\Olk"
$pkg = "$env:LOCALAPPDATA\Packages\Microsoft.OutlookForWindows_8wekyb3d8bbwe"
Get-Item "$olk\EBWebView", "$olk\UserSettings.json", "$pkg\LocalState\OlkSentinel" |
  Select-Object FullName, CreationTime

What was lost and what was not

Only local app data was deleted. Mailboxes, calendars, and contacts are stored on the Microsoft 365 and Outlook.com servers and reappear after you sign in again. Your Windows sign-in (Web Account Manager) remains intact; Outlook directly offers Microsoft accounts stored in Windows when adding an account.

You need to set up again:

• all accounts, in the desired order (the first account added is the primary account; see below)
• locally stored signatures, unless they return through the account’s cloud synchronization
• attached PST files
• app settings such as view, notifications, and startup behavior

For the primary account, the order is crucial because the new Outlook offers certain features only in the primary account, such as S/MIME (see S/MIME signature in a secondary account). Therefore, add the account that is to be the primary account first. If Outlook has already automatically adopted a different account after the reset, remove it, add the desired primary account, and then add the remaining accounts again.

For administrators

The reset cannot be undone after it occurs; backing up the folder %LOCALAPPDATA%\Microsoft\Olk before startup is the only way to restore the previous state. In environments with profile containers (FSLogix) and nonpersistent VDI sessions, you should verify that both the Olk folder and the package folder Microsoft.OutlookForWindows_8wekyb3d8bbwe are retained in the profile. If only one of the two folders is retained, that exactly matches the state the check treats as a new installation.

Reports from users that the new Outlook has “forgotten all accounts” can be classified within a few minutes using the log query above. If it says Reset sentinel check triggered user data reset, this is not an account issue or a security incident.