[Dezio]

Ok, I copy the .htaccess from do_not_upload/rewrite/apache2/forum/.htaccess to the /forum/ directory on the server.
I also edited the main .htaccess of wordpress on ./ like this:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

RewriteCond %{REQUEST_URI} !^/(forum|forum/.*)$

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

but I've the same problem :/

--------------- Added [DATE]1372703151[/DATE] at [TIME]1372703151[/TIME] ---------------

Ok, found solution:

# If you are having problems or are using VirtualDocumentRoot, uncomment this line and set 

it to your forum component directory.
RewriteBase /forum/