I've installed months ago drupal 6 and i'm still working on mods, configuration, adding contribs, to make it be what i need.

Today, for the first time, i tried loggin in with a general purpose user and tried make things. Well, this happens:

1.
If i login from the login block on my front page i have no messages back (neither notifies nor warning or errors), but the login block is still showing after the form submit, like if i did never login. Using same block with superadmin user everything works fine.

I took a look to DBLOG and what i saw was two messages in line from the user module: Session created and Session destroyed. Without any human action.

2.
I'm using a custom theme for the frontend and garland for the backend. So if i go and try login in drupalroot/admin login does work properly, and i can navigate the left tree menu.
If i go back on the front page i will automatically logout.

3.
If i try login from drupalroot/user/login it still keeps working; that's just until i go back to my front page, where my session is destroyed again.

I'm very confused, does anyone already experienced issues like this? What can destroy my session in the front page if i do not login with the superadmin?

Thank you.