[support] How to move a Drupal site below the domain root folder?

[Albert Cuesta]

As part of a Drupal 5.x to 6.x upgrade, I have moved my Drupal site to
a folder below the HTML root level of my server, so now it's at
public_html/drupal6 instead of public_html as before.

Now the site stopped working beyond the front page: users can login
but cannot logout, all links display an unthemed front page, and so on

Can somebody explain the required configuration of .htaccess (for the
domain root and the drupal6 folder) and settings.php so the site works
again? I think I have already tried everything in the handbook and
readme :-(

Thank you in advance
--
Albert