No such field 'Username'

Look at the HTML source of the login page (not the raw PHP, but the resulting HTML, e.g., in your browser use the View Page Source feature) and see what that form field is called in its name attribute. Apparently it is not called 'Username'. (Either that or the site isn't feeding the same content to Mechanize that it would give to a regular browser, but that's unlikely if it's a PHP site, IMO, especially if you wrote the PHP, because then you'd know if it was sniffing.)