error on wordpress 3.5

PHP Warning: Cannot modify header information – headers already sent by (output started at C:\sites\wordpress.procbel.com\wp-content\plugins\activehelper-livehelp\activehelper-livehelp.php:870) in C:\sites\wordpress.procbel.com\wp-includes\pluggable.php on line 876

http://ww.wp.xz.cn/extend/plugins/activehelper-livehelp/