Enable the display of PHP error messages, so that it will give you more
details than "Error 500":
https://www.mediawiki.org/wiki/Manual:How_to_debug#PHP_errors
(You will have to use the php.ini or .htaccess method, since obviously
you don't have a LocalSettings.php file yet. Ignore everything on that
page that mentions LocalSettings.php :) )
Once you have a reasonable error message, we should be able to find out
what is going wrong.
--
Bartosz Dziewoński