blank password to call logonuser function?

I am trying to authenticate users on NT networks using LogOnUser
LogonUser(txtUserNameWin.Text, txtDomain.Text, txtPasswordWin.Text, 3, 0,
TK)

I had this working for many years. Recently if failed returning
1327 : " Logon failure: user account restriction. Possible reasons are blank
passwords not allowed, logon hour restrictions, or a policy restriction has
been enforced. "

None of which is true.

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