Open Regedit and navigate to: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NTDS\Parameters Ensure the Allowed Protection Levels value is not set to an invalid state. Then, download Process Monitor from Microsoft Sysinternals. Filter on Process Name containing lsass.exe and Path containing synsoacc.dll . Look for “ACCESS DENIED” results. Restore default permissions if necessary using icalcs :
sfc /scannow This will replace any corrupted versions of synsoacc.dll from the cached WinSxS folder. If SFC reports corruption but cannot fix it, proceed to:
klist purge Reboot and attempt to re-authenticate.