Weird (and very annoying) login problem

Hello everyone

I'm having a big problem with a drupal 6 fresh website that's driving me nuts !

Everything works fine in the beginning, so i go on and install the different modules i need, add some content, test users, configure permissions, the different parameters and at some point, boom the login system goes crazy

It's bahaviour becomes random.
Here are the situation :
- It sometimes works fine
- Sometimes it just refreshes the page i was in and the login block is displayed again as if nothing happened (no error message),
- Sometimes i have an error message saying : Access denied. You are not authorized to access this page.

The thing is when i type in a wrong username or password, it displays the right error message asking if i've forgotten one or the other.
So the part of the login system that checks the login information is working fine, but the rest isn't.

The recent log entries page reports the sessions opened for the users who logged in.
So in the backend, the system considers that the users are logged in whereas in the front end, they still see the login form and they aren't able to access the content.

Even when a user tries to log out (by visiting the /logout url) an error message says Access denied.

This is the third time i'm starting over with a fresh install, and i'm going mad with this !

I should point out that i uncommented the $cookie_domain variable in settings.php so that the user remains logged in between the http://domain.tld and http://www.domain.tld

Thank you for any help !!!!