I get this error as well. I am using WordPress 3.9.1 and WooCommerce 2.1.12 and Zopim 1.3.0 and I get this error when I do a listing of orders in the WooCommerce Orders page. I have disabled Zopim and the error disappears.
Using Elegant Themes StyleShop
Warning: Cannot modify header information – headers already sent by (output started at /home/x3cmpdge/public_html/wp-content/plugins/zopim-live-chat/accountconfig.php:20) in /home/x3cmpdge/public_html/wp-includes/pluggable.php on line 1121
