Are you using suPHP on your server ? If yes please make the following configuration in Softaculous Admin panel :
http://softaculous.com/docs/SuPHP_Settings and if you are not using suPHP then the values in the above fields should be blank.
It was also because he had a custom .htaccess for his main domain. He had the forums installed in an add-on domain and maybe that's why?
I just removed the .htaccess from his main domain and everything worked fine.