Woocommerce error: "cannot modify header information… on index.php:10

Absolutely dying on this one. Any help would be supremely appreciated!

My site:
http://robinberzinmd.com/detox/ (wordpress 4.0 + woocommerce 2.2)

Issue:
Click the woocommerce “add to cart” button and see the following page “http://robinberzinmd.com/detox/?add-to-cart=272” open with the following error:

Error:
Warning: Cannot modify header information - headers already sent by (output started at /home4/dberzin/public_html/robinberzinmd.com/index.php:10) in /home4/dberzin/public_html/robinberzinmd.com/wp-includes/pluggable.php on line 1173

I’ve read all about blank spaces around php tags, but this does NOT seem related.

1. this works without any issue on my staging site (wtf!?)
2. the error is within index.php line 10. The content of index.php are simply:

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */
/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);
/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . '/wp-blog-header.php' );

I am completely stuck and have searched google up and down without any clues related to this specific woocommerce issue.

One more thing:
– http://robinberzinmd.com/cart/ works fine and shows that the above clicks do in fact add the items to car, but clicking “proceed to checkout” from the /cart page results in the same error above.

Pls hlp!
THANKS!
[D]