There was no .htaccess file. I created one and put this inside.
<IfModule mod_rewrite.c>
Options -MultiViews
RewriteEngine On
RewriteBase /forum/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule \.(jpeg|jpg|gif|png)$ /forum/public/404.php [NC,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
That seems to solve the problem. But I don't know if it is ok.