Two methods:
1. Using regedit:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Lsa
“LimitBlankPasswordUse”=dword:00000000
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
“LimitBlankPasswordUse”=dword:00000000
2. Using Local Security Policy:
Control Panel -> Administrative Tools -> Local Security Policy (run as system administrator)
Local Security Policy -> Local Policies -> Security Options
Set “Accounts: Limit local account use of blank passwords to console logon only” to Disable
Useful links: