I have read the documentation and some previous posts about this topic but could not find a solution. My app is in a subfolder: http://www.mindfulnessbergamo.net/codoforum. In the root I have an htaccess file with mod rewrite as follows:

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteBase /

    RewriteCond %{REQUEST_FILENAME} -f [OR]
    RewriteCond %{REQUEST_FILENAME} -d
    RewriteRule ^(.+) - [PT,L]

    RewriteCond %{REQUEST_URI} !=/favicon.ico
    RewriteRule ^(.*) index.php

    RewriteCond %{HTTP:Authorization}  !^$
    RewriteRule .* - [E=REMOTE_USER:%{HTTP:Authorization}]

    RewriteCond %{HTTP_REFERER} ^http://.*get-free-social-traffic\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} ^http://.*floating-share-buttons\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} ^http://.*event-tracking\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} ^http://.*Get-Free-Traffic-Now\.com [NC,OR]
    RewriteCond %{HTTP_REFERER} ^http://.*free-floating-buttons\.com
    RewriteRule .* - [F]
</IfModule>

What rule(s) do I have to add in order to have clean urls in my forum? Thanks for any help.

Have you made this change in sites/default/constants.php ?

Replace

define('SEF', 0); //Search engine freindly urls , 1=> enable , 0=> disable

with

define('SEF', 1); //Search engine freindly urls , 1=> enable , 0=> disable