Internal Server Error can be anything: script errors not caught by your server's PHP error handler, MySQL errors, server configuration issues.
I most often see it with a misconfigured .htaccess, maybe that file is trying to forward vault/install/upgrade.php somewhere else.
I have also seen it as a 404 error, but the server cannot find the custom 404 error page, and returns 500. In this case, your vault/install folder might be missing, empty, or doesn't contain the upgrade.php file.