Upgrade to 2.6 OK but I cannot login anymore

Hi…
I’ve just upgraded from 2.5.1 to 2.6 using WPAU plugin. Everything seemed to work perfectly until I got asked to login again… Well NO WAY to login…
I’ve tried anything:
– resetting the password (no results)
– write a new password directly via-mysql using a MD5 field (no results)
– re-installing the wp2.6 files (no results)
I always get the login page and I can’t access to the admin panel.
Please note that my ScribeFire (firefox plugin) still works perfectly, so the xmlrpc interface is up and running and the authentication is correct.

The login page always shows a system message asking to re-enter my log in data… even if it’s the first time I reach the page.

Any clue?

My blog is: https://www.totanus.net/ (italian language)

Thank you everyone who will help me…
Matteo