As suggested by Microsoft, you can disable this blank password restriction policy by:
1. Click Start, point to Run, type gpedit.msc, and then click OK to start the Group Policy Editor.
2. Open Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options\Accounts: Limit local account use of blank passwords to console logon only.
3. Double-click Limit local account use of blank passwords to consol logon only.
4. Click Disabled, and then click OK.
5. Quit Group Policy Editor.
