ok I figured the problem out and I hope it's useful for the plugin developer
in my setup I'm using Wordpress as CMS and I'm not using the standard blog function and index.php is empty
however, if I use TML as a function to display the form somewhere on my page (header) and if there's a login failure (wrong pwd, user name, blank field, etc), the form will redirect to index.php and not to front-page.php which is the wordpress home page and website root
I had to put an include inside index.php to call front-page.php whichh fixed the issue, because TML was erroneusly redirecting to index.php even if I was executing the form from front-page.php