Looks like the site made a small change to the login form, Updating het_browser seems to fix this both with and without a username being specified.
Code:
def get_browser(self):
br = BasicNewsRecipe.get_browser(self)
if self.username is not None and self.password is not None:
br.open('https://lwn.net/Login/')
br.select_form(name='loginform')
br['uname'] = self.username
br['pword'] = self.password
br.submit()
return br