.htaccess is causing 500 internal server error.

Sitellite .htaccess does not work with suPHP-based web hosts out of the box. Here is an exerpt from a LunarPages support article:

Your .htaccess file has php_values or php_flags in it. This causes a 500 Internal server error when attempting to execute the script.

The php_values and php_flags will need to be removed from your .htaccess file. Take the contents removed from .htaccess and place it into a file you create called php.ini. Remember to remove the php_flag and php_value part before the directives as php.ini files do not require those in front of the values.

Because php.ini values are not shared across directories, you would need a separate php.ini file in each folder that has .htaccess or that requires the php_values or php_flags. In order to avoid doing this, you can place a line in the .htaccess file to have all values in your public_html php.ini to be shared across all folder. This line would be the following:

suPHP_ConfigPath /path/to/document/root