This is because the "mod_security" plugin for apache that had been installed by your server to prevent hacking attempts is blocking access.
Your server has basically blocked the xmlrpc.php file from being accessed at all instead of using proper rules for blocking only the hacking attacks while allowing valid accesses.
However, not all hope is lost. I have "fixed" this issue with a workaround for now. It is pretty simple really. Just rename your xmlrpc.php (situated in site root, accessed at
http://yoursite.com/xmlrpc.php ) to something else (e.g. myrpc123.php). Make sure you do not add "xml" or "rpc" to the new file name you choose.
Then, in ur configuration page, select wordpress 2.2+ as ur cms and enter the path to post file as
http://yoursite.com/newlynamedfile.php Now, everything should be fine and there should be no errors.
Happy posting.
- I Speak My Word At Gravity Blue 
406 Not Acceptable
