BIG FAT ERROR after upgading to Drupal 6.8! Fatal error: Call to undefined function user_access()

I´ve tried in a testing environment, to upgrade my site from 5.x to 6.8.
I´ve followed all directions (they are almost the same as the ones that I have to go through to upgrading my site from 4.7 to 5.x).
Anyway, whan I enter the update.php file, I get this error message:

• warning: Cannot modify header information - headers already sent by (output started at /....../demo5/includes/database.mysqli.inc:128) in /......../demo5/includes/common.inc on line 141.

I´ve alreay tried deleting all possible blank spaces at the end (or top) at all the php files in the primary directory (the ones besides index.php), and deleting the blank space that were at the last line in common.inc and database.mysqli.inc

Nothing happened, but I decided to go through the upgading process anyway.

This errors appeared:

•	warning: Cannot modify header information - headers already sent by (output started at /.............../demo5/includes/database.mysqli.inc:128) in /............./demo5/includes/common.inc on line 141.
•	user warning: Duplicate entry '1850-0' for key 1 query: ALTER TABLE locales_target ADD PRIMARY KEY (language, lid, plural) in /.............../demo5/includes/database.mysql-common.inc on line 374.
•	user warning: Duplicate entry '76-14' for key 1 query: ALTER TABLE profile_values ADD PRIMARY KEY (uid, fid) in /............./demo5/includes/database.mysql-common.inc on line 374.

When I try to enter the main page or the admin page, a blank page appears with this error:

Fatal error: Call to undefined function user_access() in /............../demo5/includes/menu.inc on line 2418

Now, this is what it is at that line:

function _menu_site_is_offline() {
  // Check if site is set to off-line mode.
  if (variable_get('site_offline', 0)) {
    // Check if the user has administration privileges.
    if (user_access('administer site configuration')) {  <<<<==============ERROR LINE
      // Ensure that the off-line message is displayed only once [allowing for
      // page redirects], and specifically suppress its display on the site
      // maintenance page.
      if (drupal_get_normal_path($_GET['q']) != 'admin/settings/site-maintenance') {
        drupal_set_message(l(t('Operating in off-line mode.'), 'admin/settings/site-maintenance'), 'status', FALSE);
      }
    }
    else {
      // Anonymous users get a FALSE at the login prompt, TRUE otherwise.
      if (user_is_anonymous()) {
        return $_GET['q'] != 'user' && $_GET['q'] != 'user/login';
      }
      // Logged in users are unprivileged here, so they are logged out.
      require_once drupal_get_path('module', 'user') .'/user.pages.inc';
      user_logout();
    }
  }
  return FALSE;
}

I´ve opened wit Notepad++, and that line appears in geen, as if it were commented (but it´s not!). I don´t know if this is important.

What can I do? Please help me out!

Thanks!!!!!!

Rosamunda