How to discover cause of 403 error?

I installed antibot htaccess Rewrite rules and aprox. from that time, im getting on my PHPBB forums ocassionally 403 errors when editing some of my old posts. When edited post submitted, i get standard 403:

You don't have permission to access /posting.php on this server.

Interesting is that not all the editting cause 403 error. I dont know why.

Here are my antibot htaccess block:
http://www.askapache.com/htaccess/blocking-bad-bots-and-scrapers-with-htaccess.html#Blocking_Bad_Robots_Web_Scrapers_Rew riteRules

I also have this in my htaccess:
<Files 403.shtml>
order allow,deny
allow from all
</Files>

But i dont think htaccess is the cause, because when i rename it to: "aaaa" and try to resubmit edited post, 403 stays.

What can be cause, any way to turn on debuging mode for 403?
Im on shared hosting account, cpanel.