Outlook New reset after Windows update: accounts gone, cause in the log

After restarting for the Windows updates from 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, as were the other accounts, signatures and personal settings. There was no error message; Outlook immediately displayed the wizard for adding an account.

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 intervention by Windows Update, a user or an attacker.

Sequence of events

The timestamps in the Olk log and in the log filenames are in UTC. In the table, they have been converted to local time (CET).

The entries in the log

The first lines of the new session show the check and the 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, Last update date file missing, creating it. Barring clearing local state, this is likely a fresh install appears. From this point onwards, Outlook behaves as if it were 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. When starting 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 interprets this as an indication that the app has been reset or reinstalled and deletes the user data. On the affected device, OlkSentinel was newly created at 18:41:56; the other files in LocalState date from September 2026. The package folder itself was therefore not reset; the marker file simply did not exist before because the check was new. On the first startup with the Flight enabled, the result was therefore a reset even though none was necessary.

According to the logs, the restart by Windows Update is only the timing, not the cause: the Flight is evaluated at startup, and the first startup after the update was the first one 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 the check’s entries:

$logs = "$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 several days ago, the entry may only be in the archive and must be extracted first.

Creation times are suitable as 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 is lost and what is not

Only local app data was deleted. Mailboxes, calendars and contacts are stored on the server at Microsoft 365 and Outlook.com and reappear after signing in again. Sign-in to Windows (Web Account Manager) remains in place; 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 saved signatures, unless they return through the account’s cloud synchronisation
• attached PST files
• app settings such as view, notifications and startup behaviour

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

For administrators

The reset cannot be undone after it has occurred; a backup of the %LOCALAPPDATA%\Microsoft\Olk folder before startup is the only way to restore the previous state. In environments with profile containers (FSLogix) and non-persistent VDI sessions, you should check whether 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, this exactly corresponds to the state that the check interprets as a new installation.

Reports from users that the new Outlook has “forgotten all accounts” can be classified within minutes using the log query above. If Reset sentinel check triggered user data reset appears there, this is neither an account problem nor a security incident.