Thanks for the great plugin.
I have WordPress in a sub directory in my site and the main index.php in the root directory. This caused the path to the CSS file to be incorrect.
Solution: edit syntax_hilite.php:
around line 32, REPLACE
$igsyntax_hiliter_path = get_settings('home')
with
$igsyntax_hiliter_path = get_settings('siteurl')