Ok I solved it by putting in the .htaccess file
<IfModule mod_security.c>
# Turn off mod_security filtering.
SecFilterEngine Off
# The below probably isn’t needed, but better safe than sorry.
SecFilterScanPOST Off
</IfModule>
right after # Rewrite directives here for SEF (Search Engine Friendly) URLs
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d