The current version doesn't support sites which uses SSL.
WP_CONTENT_URL is usually initialized with the http protocol, cause
get_option("siteurl") returns this. Replacing line 67 in wp-syntax.php with
$css_url = site_url('wp-content/plugins/wp-syntax/wp-syntax.css'); fixes the problem on my side. Would be nice if it could be added to the next version.