Permalink is not working with subcategory

Hello all,

I have installed wordpress 3.5.1 and everything is fine but when I changed the permalink to be /%category%/%postname% and in my post I choose a child category the post will be blank page. If I choose the parent category it works fine. If I change the permalink to /postname works too but I like to have my category/and subcategory in my url.

Could somebody help me with fixing that please? I googled some topics and tried disabling some plugins and changed my .htaccess but nothing worked.

Thanks,

I found that the sub category page is also giving 404 error ( file not found )