I got the same error with WP 2.5.1
Make a new text file called htaccess.txt
Put this in it
<IfModule mod_security.c>
SecFilterInheritance Off
</IfModule>
Upload it to the root of your WP installation, then change its name to .htaccess
It worked for me.
Best wishes
Rob
(www.blog.kniffty.co.uk)
