session_start Error - e-newsletter

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /var/www/vhosts/example.com/httpdocs/wp-content/plugins/bp-xtra-signup/bp-xtra-signup.php:146) in /var/www/vhosts/example.com/httpdocs/wp-content/plugins/e-newsletter/e-newsletter.php on line 1464

What is this error?