after upgrading to EE 2.9.0 any valid user login attempt to CP or front end is not sucessful

Question

I've seen posts like this but I've reached a dead end, can anyone help ?
Post upgrade from 2.8.1 to 2.9.0

1. logging in as front end user:
   First thing I see is that "S=0" is prefixed before the template segments, like this http:///?/S=0/account/login
   This S=0 was not present at previous version EE2.8.1 Core.
   The login form is functioning, as a POST with an incorrect password is picked up correctly.
   A correct set of details merely refreshes the screen and clears out the form fields.

2. logging in as CP user
   No "S=0" this time ; http:///admin.php?/cp/login As for front end, submitting a correct set of details merely refreshes the screen and clears out the form fields.

Here's the parameters in play for both cases that may be relevant ....

Cookies only are set to on for
$config['admin_session_type] = 'c'
$config['user_session_type] = 'c'
and...
this issue occurs even when cookie_prefix and cookie_domain are set or are null

The above occurs even when
all sessions are deleted from exp_sessions;
cookies are deleted from browsers - have run this from Safari, Chrome and Firefox;
$env_config['expire_session_on_browser_close'] = 'y' and $env_config['cookie_secure']= 'n';

re modules, having seen other posts:
I don't have Zoo Flexible Admin installed;
I have disabled in the database the cp_member_login hook for the structure addon;
Also I can confirm that the "redirect after login" parameter in structure is set to 'n';
and config item allow_extensions is 'n'

thanks