An internal server error is usually caused by something in Sitellite's .htaccess file. What you can do to figure out which line it is is comment them out one by one and see if it removes the error.

Here's a copy of the .htaccess file along with expanded comments that explain what each line does:

# Make sure PATH_INFO is available in Apache 2
<IfDefine APACHE2>
   AcceptPathInfo On
</IfDefine>

# Let Apache know that 'index' is really a PHP script in disguise.
<Files index>
   ForceType application/x-httpd-php
</Files>

# Let Apache know that 'sitellite' is also a PHP script in disguise.
<Files sitellite>
   ForceType application/x-httpd-php
</Files>

# Make Sitellite the directory index handler (instead of index.html or
# index.php).
DirectoryIndex index index.html index.php

# Instruct Apache to treat XT templates as HTML files upon direct access.
# Used for previewing templates directly in the browser.
AddType text/html .tpl

# Short tags are generally undesirable, and Sitellite seeks to
# avoid using them in included libraries and other places.
php_flag short_open_tag off

I would recommend starting at the bottom and working your way up in commenting out each line. The most likely troublemakers are the last few. If it's the AddType or php_flag lines that are causing the error, simply leave them commented out. They aren't crucial to the general operation of the site and shouldn't have any generally noticeable impact if missing.