Problems with ajax

Now I am seeing a change when I turn off csrf. In the Console Net window, instead of '403 Forbidden' I'm getting '500 Internal Server Error'. There is still a csrf prob as I would obviously want to have it on but the server error is presumably on top of that. Is it perhaps an .htaccess issue? Here is my .htaccess code:

Code:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$  index.php?/$1 [L]

Incidentally, this is only an ajax prob - I can still POST with a form.