SSO to joomla using external form

You are correct,

I am trying to host a logon page to an external web server and use AD to authenticate. The main goal is to achieve SSO between different applications.

I have added the logging you suggested and, as expected, the username is correct. I replaced external sso form with a standard login form but again a i can't login to joomla.

So, i believe that SSO mapping is not working as expected and credentials are not posted to joomla.