mod_security and error.log

Here is the problem...
Can someone explain what is this ?
http://www.kanenas.net/comments.php?y=06&m=10&entry=entry061005-143857
I am linking to my blog because when I tried to post to Nuxified.org, I got this...

Not Acceptable
An appropriate representation of the requested resource /node/add/forum/0
could not be found on this server.

Thanks !