Subdomain crashed when installing SEO4SMF on main domain

Hello guys,

Just these few days, i try to make some blog in a new subdomain.
My main domain is http://if.web.id

Btw, our server specs:
OS: Linux
PHP: 5.2
MySQL: 4.4

I try to make a subdomain: http://boy.if.web.id

All files on my domain is put on: home/cyclon/public_html/

When i make newdomain , it will create a new folder: home/cyclon/public_html/boy/

Automatically, this new subdomain will acces all files in this new folder.
That is the problem, some files contains http://....action=... on this new subdomain, when its executed, it will raise an error, an internal error 500.

I know the problem is in the .htaccess file, i dont understand about htaccess, i just doing some trial n error, keep learning about this htaccess until maybe one or some of you give me some solution.

Thanx.