I'm not sure how to enable PHP on that index page.
Just changing the filename to index.php will do it, and it'd be useful to know if it works!
But, back to the redirect...
Well, I can't see anything wrong with the code you've posted. It looks like a CGI related error, but I'd need to see those error logs to know what's what. I'll keep thinking - hopefully in the meantime more help'll be along...
