Cannot modify header information - WHMCS WordPress Integration

hi, am trying to install WHMCS WordPress Integration but when ever i install it, i have an error on the bridge page:
Warning: Cannot modify header information - headers already sent by (output started at /home/ramielian/public_html/wp-content/themes/ainex/header.php:2) in /home/ramielian/public_html/wp-includes/pluggable.php on line 1179
i tried disabling all plugins but the problem is when this plugin is enabled..
thanks